By Fredreka Schouten, CNN

(CNN) -- Two Democratic super PACs are launching a $7.5 million advertising campaign on Monday to encourage voters to request mail-in ballots in several states crucial to their party's hopes of winning the White House and flipping control of the Senate.

The new campaign by Priorities USA Action and the Senate Majority PAC, reported first by CNN, marks the first major advertising push by these deep-pocketed groups to encourage Democratic voters to cast mail-in ballots and is part of a larger, multi-pronged vote-by-mail messaging effort by Democrats in the run-up to Election Day.

Tens of millions of Americans will vote by mail for the first time this year, as states race to offer voting alternatives in response to the coronavirus pandemic. Thirty-five states now allow all voters to request a mail ballot for the general election. In addition, nine others and Washington, DC, will automatically send a ballot to registered voters.

But voting by mail, has become the source of intense political and legal battles.

President Donald Trump has criticized mail-in voting as ripe for fraud, even as his campaign apparatus works to reassure Republican voters that the practice is safe. And the President's criticism of vote-by-mail and operational changes at the Post Office brought about by Trump-aligned Postmaster General Louis DeJoy have sparked Democratic accusations that the President's allies are trying to hamstring the US Postal Service operations before November's general election.

Democrats have stepped up their messaging on voting by mail. During last week's virtual Democratic National Convention, speakers -- such as former first lady Michelle Obama -- implored supporters to cast their ballots early and follow up to make sure mail-in ballots were received.

The new Democratic ads, which will run in Arizona, Florida, Maine, Michigan, North Carolina, Pennsylvania and Wisconsin, will target people who have a history of only voting in person, new voters and voters of color. Much of the advertising will appear on Facebook and Instagram with the goal of reassuring voters that mail-in voting is safe, Priorities officials said.

The ads also drive viewers to sites with specific instructions on how to request a mail-in ballots in voters' individual states.

One ad, called "This Isn't a Mailbox," seeks to draw a connection between Black Lives Matter protests and the act of voting. It opens with young people on the streets protesting the death of Black people at the hands of police as a narrator says: "This isn't a rebellion; it's an uprising. This isn't a moment. It's a movement."

It ends with an image of someone inserting mail in a sidewalk mailbox as the narrator concludes: "And this isn't a mailbox. It's a ballot box."

Once people have requested absentee ballots, the groups will conduct "ballot chase" programs that target those voters with digital ads that encourage them to return the ballots.

"Donald Trump's mismanagement of the coronavirus and continued campaign of misinformation about the security of our elections has created unprecedented challenges for millions of Americans to access the ballot in November," Priorities chairman Guy Cecil said in a statement. He said his group wants voters to "have their voices heard, even if they can't leave their house."

Trump campaign officials noted that Obama also encouraged people to cast early votes in person, if they could, during her keynote address.

"Joe Biden and the radical left are trying to distract American voters from their efforts to shred election integrity provisions and radically change the way America votes 70 days out from a presidential election -- all while even Michelle Obama encourages in-person voting and standing in line all night to do so," Trump campaign spokeswoman Thea McDonald said in an emailed statement. The President's campaign, she said, "is working to ensure every voter knows how their state's voting system works, and President Trump is fighting for a free and fair election and for every vote to count -- once."

The new ad campaign from Priorities is part of a previously announced $24 million voter-mobilization effort by the Democratic group. Priorities also has said it is spending $32 million on voting-related litigation.