NEW YORK (CNNMoney) -- Saturday, April 29, will mark President Trump's 100th day in office. But will it also mark Day 1 of a government shutdown?

It will if Congress doesn't pass a spending bill soon that authorizes funding for the federal government -- and if the president doesn't sign it.

The current temporary spending bill -- a so-called continuing resolution -- expires on April 28. But there are still five months remaining for this fiscal year that need funding.

It certainly won't look good if the government shuts down on Republicans' watch, since they control both Congress and the White House, especially given questions about their ability to govern in the wake of the party's recent failure to accomplish its No. 1 goal: Obamacare repeal.

"Republicans do not want a shutdown. Oh my God, they don't want that," said Steve Bell, a former senior staffer on the Senate Budget Committee.

Indeed, Senator John Cornyn, the assistant Republican leader in the Senate, said point-blank to reporters this week, "There's not going to be a shutdown."

But nothing is easy with the Trump administration or with budget debates in a highly divisive Congress.

Here are several issues that could stymie efforts to avert a shutdown by April 28:

Wall funding: If Republicans try to authorize funding for Trump's controversial border wall, that will be a no-go for Democrats.

"That will not stand," Senate Minority Leader Charles Schumer has noted.

As of this week, anyway, there are hints that top Republicans won't push to include money for the wall in the government funding bill, but may choose to under a separate vehicle.

Defense spending hike with domestic program cuts: President Trump is requesting a $30 billion increase for defense and homeland security. To offset that, he's proposing $18 billion in cuts to non-defense programs, which make up the smallest part of the federal budget.

That's also not going to work for Democrats, who want parity in spending levels for both defense and nondefense.

On the other hand, not increasing defense spending could become a major sticking point for defense hawks.

Planned Parenthood: Republicans were planning to strip funding for the health care services organization in their Obamacare repeal bill. Since that bill is dead for now, conservatives may push for a defunding measure in the CR. If they do, bye-bye Democratic votes.

"Speaker Ryan signaled it will not be in the House bill, though that will cost many GOP defections on the bill. That means the Democrats have even more leverage," Chris Krueger, a senior policy analyst at the Cowen Washington Research Group, said in a morning note.

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