President Donald Trump is threatening to shut down the government over immigration — for the third time this year. And this time, if he followed through, it would happen just a little over a month before the highly contentious midterm elections.

The government runs out of money at the end of September, a deadline congressional Republicans are eager to meet in order to avoid any additional drama before the November elections. But that’s been made all the more complicated by Trump, who nearly vetoed a spending package earlier this year because it didn’t fund his border wall, and is now threatening to do so again with even more sweeping hardline immigration demands.

On Sunday, Trump tweeted that he would again be willing to veto a spending bill if “Democrats do not give us the votes” on border security, so-called “catch and release,” and the diversity visa lottery.

I would be willing to “shut down” government if the Democrats do not give us the votes for Border Security, which includes the Wall! Must get rid of Lottery, Catch & Release etc. and finally go to system of Immigration based on MERIT! We need great people coming into our Country! — Donald J. Trump (@realDonaldTrump) July 29, 2018

Trump’s tweet is a considerable blow to House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell, who visited the president last week to pitch him on leaving immigration out of this spending fight. Trump was reportedly receptive to their pleas last week — but not for long.

Republicans are facing a competitive midterm election cycle, in a year when Democrats are gunning to retake control of the House and even possibly make gains in the Senate. With Trump’s low approval rating and a somewhat short list of legislative wins to tout on the campaign trail, the last thing Republicans want is to have to defend an unpopular government shutdown over Trump’s already unpopular immigration agenda.

Trump doesn’t just want a wall. He’s also asking for two very politically contentious and sweeping immigration reforms.

Trump’s demands are significant, and they cover issues that Congress has shown time and time again they can’t agree on. While the tweet is vague on policy specifics, Trump is essentially calling for three sweeping immigration reforms.

1) Funding for the border wall. This is one Trump’s longest-standing asks on immigration. He’s demanded everywhere from $5 billion to the full $25 billion to build the wall. (Multiple reports have found that building the wall will actually cost much more than that.)

Democrats have been in lockstep against funding the wall from the start, but they’ve also been clear that they are willing to concede border wall and border security funding if Trump will agree to a legislative fix for the Deferred Action for Childhood Arrivals program. The president, however, has rejected every Democratic-sponsored offer so far. Instead, he has espoused hardline views on immigration that Democrats won’t agree to and don’t even have consensus among Republicans.

2) An end to “catch and release.” “Catch and release” is what Republicans call the practice of releasing undocumented immigrants from detention and allowing them to live in the US while they wait for their asylum claims to be adjudicated. Trump’s administration has attacked this practice from the start, claiming it’s allowed for “widespread lawlessness,” as Vox’s Dara Lind explained. The Trump administration’s attempt to end “catch and release” is what ultimately led to the “zero tolerance” border policy and the separation of more than 2,500 immigrant children from their families.

The bipartisan backlash against family separations ultimately pushed the administration to reverse its policy — but the issue is not resolved. To fully address family separations, conservatives and Trump allies are calling for Congress to pass a law overturning a court order that prevents children from being detained indefinitely with their families at the border. This isn’t something lawmakers, past Trump’s most conservative allies on Capitol Hill, agree on.

3) Sweeping reforms to legal immigration. Every immigration proposal the White House has offered so far has demanded major changes to the legal immigration system — from getting rid of the diversity visa lottery program to ending family-based immigration. The idea is that people should only be allowed to enter the US if they demonstrate a valuable asset, instead of just being sponsored by their relatives. But Trump’s proposed reforms have all substantially cut the number of legal immigrants that would be allowed into the country — a reform that is a nonstarter for Democrats and one that traditional Republicans are also uncomfortable with.

The bottom line is Trump is not backing down from demands for comprehensive immigration reform, something the Senate and House have tried and failed on both a bipartisan and partisan basis.

Now Trump is raising the stakes, tying it to a highly contentious spending fight mere months before an election.

Trump’s immigration agenda is very unpopular. And he keeps forcing it on Republicans.

If it were up to Republicans, Congress wouldn’t be dealing with immigration right now — and they certainly wouldn’t be advocating for a government shutdown in the months leading up to the midterms. Lawmakers typically avoid divisive issues in an election year.

For the past five months, the House and Senate have been sprinting to pass all 12 of their annual spending bills before the September deadline. The House has passed six funding bills, and the Senate is on the path to pass nine by the end of next week, leaving the most contentious spending battles over the Department of Homeland Security for later in the summer.

Republicans know government shutdowns are very unpopular. And Trump’s immigration agenda doesn’t fare well among voters overall either. More than half of Americans — 58 percent — disapprove of how Trump has handled immigration matters, according to an early July Quinnipiac poll. A Reuters poll similarly found 52 percent of Americans disapproved of Trump’s immigration agenda. Republicans are much more likely to approve of Trump’s immigration agenda, but a government shutdown over immigration policy could be disastrous for vulnerable Republican candidates.

But even in a year when Democrats are trying to harness their base’s enthusiasm to knock Republicans out of power in Congress, Trump hasn’t heeded his party’s political concerns. From his border wall to the administration’s decision to sunset DACA to the family separations at the border, Trump has forced Congress to address some of the most contentious immigration policies without much direction or planning. He’s also refused to back down from hardline anti-immigration demands that don’t even have the support of his entire party.

Congress keeps failing to find consensus on the narrowest of immigration reforms. And yet Trump keeps trying to force their hand on the most contentious comprehensive reforms.