Senate Minority Leader Chuck Schumer has a commitment from Republicans to see a Senate debate on the future of nearly 700,000 immigrants currently protected by the Deferred Action for Childhood Arrivals (DACA) program in the next three weeks. But the real hurdle for immigration reform is in the House of Representatives.

It all comes down to a promise Rep. Paul Ryan (R-WI) made in 2015, before taking over the speaker’s office: He assured the most conservative members of his conference — the very ones who ousted his predecessor John Boehner — that he would not let an immigration bill pass without the support of the majority of House Republicans.

It’s called the Hastert Rule, an unwritten Republican leadership principle named after former Republican House Speaker Dennis Hastert. (Yes, the same Dennis Hastert who went to prison for a bank fraud conviction related to him molesting young boys.) And it has dictated how the immigration debate has lived and died in Congress over the past decade.

At the height of the government shutdown, Senate Democrats were demanding assurances from Ryan and President Donald Trump that a bipartisan DACA deal would make it not only out of the Senate but through the House and to the president’s desk. By the end of the shutdown, these calls had ultimately quieted.

But Democrats’ concerns still stand, and as the immigration debate gets underway in Congress, the political headwinds in the House will likely prove to be the biggest test.

A brief history of the Hastert Rule

Hastert, the longest-serving Republican speaker of the House, coined the “Hastert Rule” in 2003, when he said the “job of the Speaker is not to expedite legislation that runs counter to the wishes of the majority of the majority.”

In other words, speakers shouldn’t allow a vote on legislation unless the majority party is unified behind it. As George Washington University political scientist Sarah Binder notes, this “rule” predates Hastert and “reflects a decades-long pattern in the House of more aggressive Democratic and Republican majority party leaders.” Binder writes:

The basic premise of the rule — that House leaders will use their leverage over the floor agenda to keep measures off the floor that might divide the majority party — has guided House majority party leaders at least since the early 1980s.

When the “majority of the majority” rule is violated, it’s usually on must-pass pieces of legislation, like spending bills or emergency disaster relief funding.

In 2004, Hastert’s congressional aide John Feehery defended the rule to the Washington Post, saying, “If you pass major bills without the majority of the majority, then you tend not to be a long-term speaker.”

That turned out to be the case for Speaker Boehner, who, leading a splintered Republicans caucus, angered House conservatives by seeking votes across the aisle to pass major pieces of legislation — a move that ultimately led to his ouster. He broke the Hastert Rule over the fiscal cliff, the debt limit, and relief funding for Hurricane Sandy — national emergencies that threatened economic calamity and American lives.

But even Boehner used the “Hastert Rule” as political cover, to prevent contentious pieces of legislation from dividing his party and to limit the power of the minority. This includes immigration reform: He refused to bring the Senate’s bipartisan “Gang of Eight” comprehensive immigration reform bill up for a vote in the House in 2013, saying it wouldn’t receive the support of the “majority of the majority.”

Ryan, who came into the speakership on the tail of an archconservative revolt, has tried to keep the peace (to varying degrees of success) within his party. Before taking the speakership, he promised the conference’s most conservative members, who were most wary of his appointment, that he would not move on immigration without the support of the majority of the caucus.

The Hastert Rule could make or break a DACA deal

Boehner’s refusal to bring up the “Gang of Eight” immigration bill for a vote is a fresh memory for Democrats who want assurances that whatever bipartisan immigration bill is finally given a vote, and passed, in the Senate will then make it to the president’s desk.

But the Hastert Rule could pose a major obstacle. Already several Republican lawmakers have speculated it would be near impossible to reach 120 Republican votes — the majority of the GOP conference — on an immigration bill in the House.

Rep. Charlie Dent (R-PA), who chairs a group of moderate Republicans in the Tuesday Group, says the Hastert Rule has been holding up negotiations. He expects that “any bipartisan deal on DACA will likely not receive the majority of the majority votes.”

“The majority of the majority — if I hear that one more time, my head is going to explode,” Dent said. “We are all for the majority of the majority on the bill until we are not.”

But for now, Ryan has only doubled down on his commitments to conservatives. During the House’s government spending fight last week, Ryan promised the House Freedom Caucus, a group of 40 ultraconservative lawmakers, that Republican leadership would whip votes for a conservative, partisan immigration bill.

Meanwhile, Trump, who continues to engage immigration hawks in negotiations, has assured Congress’s hardliners — like the Freedom Caucus’s Rep. Mark Meadows (R-NC) and Sen. Tom Cotton (R-AR) — that he wouldn’t approve of an immigration bill without their blessing. No proposal put forward by Meadows and Cotton has received bipartisan support.

With conservatives still at the negotiating table, Democrats want GOP leadership to attach an immigration bill to a “must-pass” spending or budget bill to ensure that it gets a vote in the lower chamber and sees Trump’s signature.

“We have to have in our own mind some way to ensure that the House feels a need to bring up the issue as well,” Sen. Dick Durbin (D-IL) told reporters.

But while Senate Majority Leader Mitch McConnell has agreed to allow for a vote on immigration in the Senate, he has been unable — or unwilling — to bind the House and Trump in that process. And outside of assuring Democrats that Republicans are negotiating immigration in earnest, Ryan has made no promises.

There are a lot of immigration proposals. None have enough support.

Before the government shutdown, House Minority Leader Nancy Pelosi reportedly asked Ryan to hold votes on two immigration bills and see which one passed: a bipartisan agreement put forward by Reps. Will Hurd (R-TX) and Pete Aguilar (D-CA), and Rep. Bob Goodlatte’s (R-VA) conservative proposal.

Ryan refused — likely because the bipartisan one would pass without the majority of the majority. As Vox’s Dara Lind and I explained, the problem in Congress right now isn’t a lack of proposals on immigration; it’s a lack of the right kind of support:

It’s that the ideas that have been presented are either too far to the left to satisfy “a majority of the majority” among House Republicans or too far to the right to attract any Democratic votes in the Senate.

In the Senate, there is an agreement between Sens. Lindsey Graham (R-SC) and Durbin that would offer legal status, and eventual citizenship, to young unauthorized immigrants, give about a year’s worth of funding for the border “wall,” and eliminate the diversity visa lottery, but it was panned by the White House and conservatives for being too liberal. There are a handful of other bipartisan proposals that follow a similar framework, including the Hurd-Aguilar proposal in the House.

Meanwhile, the Goodlatte bill — which conservatives and Trump have coalesced around — would criminalize all unauthorized immigrants in the US and curb legal immigration by 25 percent, while giving legal status to current DACA recipients, and is too far to the right to garner any Democratic votes in the Senate.

Either option runs the risk of failing the Hastert Rule.

“This [Goodlatte] proposal is designed not to become law — everyone knows that,” Dent said. He argues that a vote on a partisan bill would only give more Republicans in the House an excuse to vote against a bipartisan agreement.

“The Goodlatte bill will incentivize people to vote against the final product on the Republican side,” he said.