Updated: 12:48 a.m.

House conservatives stymied Speaker John Boehner’s debt limit bill Thursday night, forcing leadership to pull it from the floor rather than risk losing a vote on passage.

It’s uncertain when the measure might return to the floor. The Ohioan and other GOP leaders had been leaning on aggressive lobbying to wrangle the 216 votes needed, but by late Thursday it was clear that enough conservatives were unwilling to budge from their opposition that the bill would go down.

The leadership is now scrambling to also come up with changes to make the bill more palatable. The Rules Committee approved a rule late Thursday that would allow Boehner to make changes to his bill and immediately bring it to the floor anytime from Friday through Tuesday, the day the Treasury Department predicts the United States will begin defaulting on its obligations if the debt ceiling isn’t raised.

The setback creates a key challenge for Boehner’s speakership and pits a results-oriented leadership team against an ideologically rigid minority that holds sway over a significant portion of GOP lawmakers.

House Republicans were scheduled to meet Friday morning, the fifth such meeting this week. But it was unclear what changes Boehner could make to his bill that would entice conservatives, aides said.

House Democrats, who pledged not to give Boehner any votes to get his bill over the hump, celebrated Thursday night’s postponement.

Minority Leader Nancy Pelosi, who earlier in the day promised that Boehner’s bill “will not win with Democratic votes,” said the evening’s developments proved it was time to go back to the drawing board.

“Hopefully, now the Republicans will come back to the table to negotiate a bipartisan, balanced agreement that is overwhelmingly supported by the American people,” the California Democrat said in a statement. “Republicans have taken us to the brink of economic chaos. The delay must end now so we can focus on the American people’s top priority: creating jobs and growing the economy.”

A planned 5:45 p.m. vote on the bill was postponed until later in the day when GOP leaders realized they didn’t have enough support. That set off a round of arm-twisting, with Boehner and Minority Whip Kevin McCarthy (R-Calif.) summoning a parade of rank-and-file conservatives into their offices.

But because of the GOP’s earmark ban, the leaders had little to use as leverage on their Members and were getting no closer to pushing the bill over the finish line.

At the same time, leadership began looking for potential tweaks to sweeten the bill for conservatives. Although those efforts were continuing, sources familiar with the situation said they remained stuck, and McCarthy informed reporters just before 10:30 p.m. that they were giving up on holding a vote Thursday night.

The breakdown in the legislative process is unprecedented in recent Congressional history, in large part because former Speakers such as Pelosi had greater control over their Members.

The effect of earmarks on previous difficult votes also cannot be overstated. Pelosi and her predecessors were able to use funding for pet projects as a way to muscle wayward Members into line on difficult votes. But with the GOP’s earmark ban in place, Boehner does not have that luxury.

Correction: July 29, 2011

An earlier version of this story misstated Rep. Kevin McCarthy’s party affiliation. McCarthy is a Republican.