Late on the night of Sept. 30, with the federal government just hours away from shutting down, House Republicans quietly made a small change to the House rules that blocked a potential avenue for ending the shutdown.

It went largely unnoticed at the time. But with the shutdown more than a week old and House Democrats searching for any legislative wiggle room to end it, the move looms large in retrospect in the minds of the minority party.

“What people don’t know is that they rigged the rules of the House to keep the government shut down,” Rep. Chris Van Hollen (D-MD), ranking member of the House Budget Committee, told TPM in an interview. “This is a blatant effort to make sure that the Senate bill did not come up for a vote.”

Here’s what happened.

The House and Senate were at an impasse on the night of Sept. 30. The House’s then-most-recent ploy for extracting Obamacare concessions from Senate Democrats and the White House — by eliminating health insurance subsidies for Congress members and their staffs — had been rejected by the Senate. The ‘clean’ Senate spending bill was back in the House’s court.

With less than two hours to midnight and shutdown, Speaker John Boehner’s latest plan emerged. House Republicans would “insist” on their latest spending bill, including the anti-Obamacare provision, and request a conference with the Senate to resolve the two chambers’ differences.

Under normal House rules, according to House Democrats, once that bill had been rejected again by the Senate, then any member of the House could have made a motion to vote on the Senate’s bill. Such a motion would have been what is called “privileged” and entitled to a vote of the full House. At that point, Democrats say, they could have joined with moderate Republicans in approving the motion and then in passing the clean Senate bill, averting a shutdown.

But previously, House Republicans had made a small but hugely consequential move to block them from doing it.

Here’s the rule in question:

When the stage of disagreement has been reached on a bill or resolution with House or Senate amendments, a motion to dispose of any amendment shall be privileged.

In other words, if the House and Senate are gridlocked as they were on the eve of the shutdown, any motion from any member to end that gridlock should be allowed to proceed. Like, for example, a motion to vote on the Senate bill. That’s how House Democrats read it.

But the House Rules Committee voted the night of Sept. 30 to change that rule for this specific bill. They added language dictating that any motion “may be offered only by the majority Leader or his designee.”

So unless House Majority Leader Eric Cantor (R-VA) wanted the Senate spending bill to come to the floor, it wasn’t going to happen. And it didn’t.

“I’ve never seen this rule used. I’m not even sure they were certain we would have found it,” a House Democratic aide told TPM. “This was an overabundance of caution on their part. ‘We’ve got to find every single crack in the dam that water can get through and plug it.'”

Congressional historians agreed that it was highly unusual for the House to reserve such power solely for the leadership.

“I’ve never heard of anything like that before,” Norm Ornstein, resident scholar at the American Enterprise Institute, told TPM.

“It is absolutely true that House rules tend to not have any explicit parliamentary rights guaranteed and narrowed to explicit party leaders,” Sarah Binder, a congressional expert at the Brookings Institution, told TPM. “That’s not typically how the rules are written.”

Republican staff on the House Rules Committee did not respond to multiple requests for comment. But here’s what House Rules Chairman Pete Sessions (R-TX) told Rep. Louise Slaughter (D-NY) when she raised those concerns before the rule change was approved.

“What we’re attempting to do is to actually get our people together rather than trying to make a decision,” Sessions said. “We’re trying to actually have a conference and the gentlewoman knows that there are rules related to privileged motions that could take place almost effective immediately, and we’re trying to go to conference.”

“You know that there could be a privileged motion at any time…,” Sessions continued as Slaughter continued to press the issue.

“To call for the vote on the Senate resolution,” Slaughter interjected. “I think you’ve taken that away.”

“I said you were correct. We took it away,” Sessions said, “and the reason why is because we want to go to conference.”