Overlay Trump's potential intransigence on a Republican Party that has long brought itself to the brink of a shutdown, and you have more factors leading to a shutdown than at any time since, well, the last one.

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“It's completely unpredictable,” said Maya MacGuineas, the president of the bipartisan Committee for a Responsible Budget. “A negotiation that one could have seen the outlines for the resolution just got a mini bomb tossed into it.”

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Here are all the reasons experts say a shutdown is more likely than not this year.

Too much to do, too little time

Congress spent much of its spring legislative session trying to pass a health-care bill. When lawmakers come back after Labor Day, they have roughly a month to:

Pass a spending bill by Oct. 1.

Lift the debt ceiling by late September so the U.S. can borrow money to pay its debts.

Fund a federal children's health insurance program.

Reauthorize a flood insurance program.

Oh, and Republican leaders want to seriously start debating a tax reform bill.

Any one of those tasks could easily take a month. And when Congress has too much on its plate, it can get paralyzed.

Republicans can't decide what should be in the spending bill

By now this is old news, but it shouldn't be overlooked. The Republican Party looks more and more like a coalition of different parties, and one of those factions has reliably refused to vote on legislation that increases government spending.

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In fact, for the 30-40-member strong House Freedom Caucus, a shutdown is like a campaign event, said Stan Collender, a nonpartisan budget expert.

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This political dynamic could also complicate the debt ceiling fight. Some uber-fiscally conservative lawmakers say they'll only vote to let the U.S. borrow more money if the U.S. promises to cut its spending.

The Trump administration is giving mixed signals on what the president would sign into law. Treasury Secretary Steven Mnuchin wants Congress just to raise the debt ceiling and leave the spending debate to another, less economically perilous piece of legislation. Trump's budget director, Mick Mulvaney (a former Freedom Caucus member), has suggested a debt ceiling raise should come with spending cuts.

Democrats can force Republicans to choose between their president and a shutdown

In budget fights past, House Speaker Paul D. Ryan (R-Wis.) has relied on Democrats to get a majority of votes for a spending bill. The same thing is true in the Senate.

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And that means Democrats have the ability to influence what's in the spending bill almost more than Republicans do. This year, their chief demand: Not a dime for Trump's border wall.

“If the President pursues this path … he will be heading toward a government shutdown which nobody will like and which won’t accomplish anything,” warned Senate Minority Leader Charles E. Schumer (D-N.Y.) in a statement Wednesday.

House Minority Leader Nancy Pelosi (D-Calif.) reminds those who want to blame Democrats for any shutdown that Republicans control Washington,

“With a Republican House, Senate and Administration, Republicans have absolutely no excuses for threatening America's families with a destructive and pointless government shutdown,” she said in a statement Wednesday.

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Democrats know full well that most Republicans don't want to be on the hook for an untold billions to build a border wall, either.

Trump is a black hole

Even with all those pitfalls, Congress has figured out how to hobble along, said Molly Reynolds, a congressional expert with the Brookings Institution. It can pass a short-term spending bill that funds the government through December to provide more time to come to an agreement.

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But, Trump. Nobody has any idea what he wants, what he'll sign, or whether he even cares if there's a shutdown.

“All he said is 'the wall,'” said Stan Collender. “We don't know whether that's bluster, we don't even know what he means by a 'wall.'”

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Trump is seven months into his presidency without a legislative win; it's entirely possible he will decide to risk it all on getting some money for his wall, now that it's clear he has no intention of forcing Mexico to pay for it.

Trump has a few options to go to war with Congress on the wall. He could veto a spending bill that doesn't have money for his wall. He could veto even a short-term spending bill. He could even veto a debt ceiling bill. On all of that, Congress could override his veto with a supermajority. Or it might not be able to.

It's all so up in the air that it's impossible to rule out a shutdown right now.