By Ellie Kaufman, Jeremy Herb and Pamela Brown, CNN

(CNN) -- President Donald Trump is expected to wage an attack on the election itself at Tuesday night's debate with former Vice President Joe Biden by repeating his past claims that conflate and exaggerate problems with mail-in voting.

The "integrity of the election" is one of six topics at the first presidential debate between Trump and Biden. A source close to the campaign said Trump's message at the debate will be consistent, and that the campaign plans to amplify any case of voter fraud or suspected voter fraud, even if it's murky. Trump will argue that even one case of fraud is too much and "integrity is one of the most important things in a democracy," the source said.

Trump also plans to go through the "laundry list of things he's done to solidify election security," according to the source, and he will respond to questions about a peaceful transition by arguing he "doesn't expect to lose."

Trump previewed his assault on mail-in voting at a Sunday press conference, ticking off issues with voting in eight states in an effort to claim widespread voter fraud due to the expansion of mail-in voting during the Covid-19 pandemic. He's continued to harp on the issue on Twitter, as he's done for months, in the run-up to Tuesday's debate.

But the examples Trump cited on Sunday show how the President is making misleading and often outright false claims about fraud related to voting, incorrectly tying the rejection of mail-in ballots to fraud, and exaggerating the impact of isolated instances involving problems with small numbers of ballots.

"We have a big problem, and you see it every day, you see it happening every day with ballots," Trump said Sunday. "When the ballots and when the system is rigged -- which it is, obviously it is -- and the only one that knows that better than me are the Democrats."

Trump's critics warn his repeated attacks on mail-in voting are undermining the integrity of the election, sowing doubts among his supporters about the results of the election.

Trump has also failed to highlight other possible threats to the election -- most notably Russia's interference efforts -- as the intelligence community says Russia is conducting an active campaign to try to denigrate Biden. And a ransomware attack targeting one of America's leading software providers has again raised concerns over the security of state and local governments' election systems ahead of the election.

Many of Trump's charges point to states where mail-in ballots have been rejected. While mail-in ballots are rejected at a higher rate than in-person ballots, that's typically not because of fraud. It's mostly due to voters failing to complete all required steps -- or often simply because the ballot was mailed too late.

Here's a breakdown of the claims Trump is making about voter fraud:

New York

"In Brooklyn, 25% of mail-in ballots were ruled invalid in June's Democrat primary. You saw that," Trump said at Sunday's press conference.

Trump has frequently harped on the issues New York faced in the primary, including delays in counting mail-in ballots. In New York's June primary, in which there was expanded mail-in voting due to the coronavirus pandemic, more than 84,000 absentee ballots were invalidated in New York City. That accounted for about 21% of all the ballots received by the city's Board of Elections, New York state Sen. Brad Hoylman said during a press conference on August 31. Hoylman was advocating for the state legislature to pass a bill that would allow the New York State Board of Elections to introduce mail-in ballot drop boxes for the upcoming November election.

But a significant percentage of these ballots were invalidated because they arrived late or without a mail postmark. Others were rejected because they were missing information -- not because of fraud. Hoylman was advocating for drop boxes so that more mail-in ballots could be guaranteed to arrive on time, rather than travel through the US Postal Service, which experienced widespread delays over the summer.

New Jersey

"In a special election in New Jersey, 20% of the ballots were thrown out, and four people are now being prosecuted for fraud," Trump said.

Trump is correct that four people were charged with voting fraud after a May 12 special election in New Jersey, when hundreds of mail-in ballots were found in a mailbox in Paterson, and numerous additional ballots were found in a nearby mailbox in Haledon, according to a press release from New Jersey Attorney General Gurbir Grewal. The election was conducted entirely by mail-in ballots because of the coronavirus pandemic.

Four individuals, including a city council member, were charged with criminal conduct involving mail-in ballots during the election. The city council member allegedly violated election laws as a candidate by approaching one or more town voters where he was running and collecting their official mail-in ballots, according to the release.

New Jersey allows a "bearer" to collect ballots for as many as three people and deliver them to the county board of elections. A candidate in the election is never permitted to be a bearer. Two of those charged allegedly violated this clause or a similar one.

While these four men were charged with voter fraud-related violations, this occurred in one isolated special election concerning city councilmembers. Trump has touted 20% of the ballots being rejected, but most of the 3,190 mail-in ballots declared invalid by the Passaic County Board of Elections were because of reasons unrelated to fraud, according to attorney Scott Salmon, who represents the incumbent city council member and alleged fraud victim. He said about 900 ballots, roughly 5% of the total vote, were potentially fraudulent.

"It's not realistic to do that on a scale that would matter for a presidential election," he said at the time. "And, frankly, you're probably going to get caught."

Florida

"35,000 mail-in ballots were rejected in Florida's primary," Trump said.

Yes, more than 35,000 mail-in ballots were rejected in Florida's August primary. These rejections made up 1.5% of the total vote in the Florida primary, according to data reported by Politico.

But just as in the other states, this was mostly due to incorrect or late ballots, not fraud: Roughly two-thirds were rejected because they arrived after Florida's 7 p.m. Election Day deadline, according to Politico, and the rest did not meet signature match requirements.

California

"One hundred thousand (ballots) were rejected in California, and that's just the beginning," Trump said.

Ballots were rejected in the primary, and that has nothing to do with fraud.

In California's March presidential primary, 102,428 mail-in ballots were rejected by California election officials, making up 1.5% of the almost 7 million mail-in ballots that were sent in, according to data from the California Secretary of State's office obtained by The Associated Press.

Missing the deadline was the most common reason these ballots were thrown out, AP found. Over 27,000 ballots were rejected because they either didn't have a signature, or the signature didn't match the one on record for the voter, according to the AP.

Pennsylvania

"A week after Pennsylvania's primary, half of the counties were still counting ballots, and you'll be counting them here because this is a much bigger version of all of that," Trump said.

Trump is correct that delays counting mail-in ballots are a big concern heading into the general election, particularly in a state like Pennsylvania that could see a close race help decide who wins the White House.

One of the biggest obstacles that states face in counting mail-in ballots is laws preventing them from beginning the tally until Election Day. Pennsylvania Secretary of the Commonwealth Kathy Boockvar, a Democrat, has pushed to allow for counting three weeks early, and Republicans and Democrats in the state legislature are still debating with Democratic Gov. Tom Wolf over allowing the count to begin early.

"Just last week, a number of discarded military ballots were discovered in Pennsylvania. All of the recovered ballots -- these were ballots that were thrown out -- had been cast for a person named Donald J. Trump," Trump said.

In Luzerne County, Pennsylvania, a contractor who had been hired to do election work improperly threw out nine absentee ballots from military personnel serving overseas, last week.

In a statement, county officials said the issue was caused by a "temporary seasonal independent contractor" who "incorrectly discarded (the ballots) into the office trash" within their first three days of work helping the elections office. On the third day, Luzerne County Elections Director Shelby Watchilla realized what had happened and opened an investigation, and the contractor "was removed from service and informed not to return."

The officials called this an "error" and said the fact that it was quickly noticed and investigated proves that "the system of checks and balances set forth in Pennsylvania elections works."

The Justice Department was involved in the case and put out a curious statement about its investigation that included a statement that the nine ballots were cast for Trump. It later corrected that statement to say seven of the nine were cast for Trump, and the other two were unknown. Attorney General William Barr briefed Trump on the investigation -- and Trump and his team quickly touted the incident.

But the claims being pushed by Trump's team are exaggerations of what appears to have actually occurred.

People briefed on the matter told CNN that federal investigators are not treating the incident as intentional fraud but rather as something that occurred because of poorly designed procedures for handling mail-in ballots, and because newly hired election workers weren't properly trained. The Department of Justice has never described this incident as voter fraud.

Wisconsin

"In Wisconsin, three trays of mail containing absentee ballots were found in a ditch. They were thrown in a ditch. Three trays. That's a lot," Trump said.

Three trays of mail that were supposed to be headed to the local post office were found on the side of the road and in a ditch off of Highway 96 in Outagamie County, Wisconsin, according to the Outagamie County Sheriff's Office, local TV station WBAY reported. Some of the mail included absentee ballots, the sheriff's office said.

Wisconsin's chief election official Meagan Wolfe said the USPS was investigating the situation. She told reporters this month that if absentee ballots were lost, they could be traced with intelligent mail barcodes placed on absentee ballot mail to ensure all ballots are tracked.

"I know that the Post Office is investigating this. We're going to keep in close contact with them as information becomes available, we'll also check in with local election officials, we'll continue to check those intelligence mail barcodes to see if there's anything happening with any ballots," Wolfe said last week. "There's still plenty of time for a voter to be reissued another ballot at this juncture."

North Carolina

"In North Carolina, voters are reporting receiving two ballots in the mail. Many -- many voters. I hear it's thousands, but they're getting two ballots. I wonder if those are Democrat areas, because the word is they are," Trump said.

The Mecklenburg County Board of Elections sent two ballots to fewer than 500 absentee voters in the middle of September. The North Carolina State Board of Elections said in a statement that the error occurred after "an election worker misplaced labels on absentee return envelopes."

Some ballots that had misplaced labels had already been mailed before the issue was identified, the state elections board said.

"To ensure all affected voters received a ballot that could be successfully returned, a new ballot was mailed to them," the board said. Those voters received two ballots because the county was trying to rectify a mistake and ensure that everyone who requested one received an admissible absentee ballot.

The problem affected fewer voters than Trump claimed, and no one who received two ballots can cast both of them.

North Carolina is using intelligence mail barcodes to track all election mail. Because of this, "the state system will not permit two ballots from the same person to be accepted or counted," the board said. Mecklenburg County has sent out about 115,000 ballots by mail as of September 17. This error affected fewer than 500 ballots.

Iowa

"In Iowa, they still don't really know who the winner was. I think they called somebody eventually, but it was many, many weeks later. But they really have no idea ... it continues to be litigated in Iowa. They can't run a simple caucus, yet now they're trying to radically write election laws nationwide, just weeks before the presidential election," Trump said.

Trump has repeatedly pointed to problems surrounding the Iowa caucuses this year to knock Democrats.

The Democratic caucuses this year were littered with problems -- results were delayed for hours amid problems with a new app to collect results and jammed phone lines.

But the method of voting in the Iowa caucuses -- where people join together and physically congregate in groups supporting their preferred candidate -- is about as far removed from mail-in voting as you can get.

Neither the Democratic Party nor the Republican Party runs state elections -- chief election officials do. And both Republican and Democratic secretaries of state have sought to make it easier for voters to cast a ballot by mail as a result of the coronavirus pandemic.