Washington (CNN) - House Republicans delayed a vote Thursday on Speaker John Boehner's plan to raise the nation's debt ceiling while enacting sweeping cuts in government spending, indicating a rift within the GOP could undermine the party's latest attempt to avoid an unprecedented national default and stave off potential economic catastrophe.

The delay showed Boehner was unable to muster sufficient support from his own caucus to guarantee his proposal would pass in the face of expected unified Democratic opposition.

It was unclear if the vote would occur on Thursday night. Influential House Budget Committee Chairman Paul Ryan, R-Wisconsin, said he expected the Boehner plan to eventually pass, but when asked about timing, answered: "I don't know the answer to that question."

"There is a delay. The reason has not been stated," Rep. Nan Hayworth, R-New York, who supports the plan, told CNN. Hayworth acknowledged that some members "have had a lot of deep thinking to do" about their votes, and said the delay might be to confirm "the last few" supporting votes.

After the announcement of the delay, conservative congressmen were seen entering and leaving Boehner's office as the speaker tried to generate the necessary support. A floor debate on the plan was cut short, and the House moved on to discuss the naming of a post office in Illinois.

Staff members were later seen carrying pizzas from a local restaurant into Boehner's office.

Defeat would be a major setback for Boehner, who assumed his post in January, and further muddy the already tense negotiations over a deficit reduction deal that would also increase the federal borrowing limit. In particular, it would show Boehner was unable to control the tea party conservatives elected last year in a Republican wave that delivered a GOP majority in the House and his ascension to speaker.

Earlier, Boehner declared to reporters that the measure would pass, but Majority Whip Kevin McCarthy, R-California, only would say progress was being made in rounding up the votes.

Another speaker at the news conference, Republican Rep. Renee Ellmers of North Carolina, let slip there's more work to be done, calling on House colleagues "who may not be there yet" but were "moving forward on those votes."

The vote in the House had been scheduled for roughly 6 p.m. ET. Few if any Democrats were expected to back the measure. Assuming House Democrats remain united against the bill, Boehner will need the support of at least 216 of the House's 240 Republicans.

Whether Boehner can push the measure through remains an open question. Tea party-backed conservatives staged a virtual revolt against the bill over the past two days, complaining that it doesn't do enough to shrink the size of government and stem the tide of Washington's red ink.