Conservative Republicans in the House were in open revolt on Thursday over their party’s major legislation to deliver President Trump’s domestic agenda, threatening to derail the tax and budget measure over concerns that it would add too much to the deficit.

In the latest indication of the dissent in the G.O.P.’s ranks, two Republicans, Representatives Chip Roy of Texas and Ralph Norman of South Carolina, told reporters they planned to vote against approving the legislation in the Budget Committee in a session planned for Friday.

Mr. Roy and Mr. Norman’s opposition alone would be enough to block the measure from reaching the floor, upending the party’s drive to push the legislation through the House before a Memorial Day recess. A number of other conservative, anti-spending Republicans sit on the panel, and suggested they could follow suit.

“Right now, the House proposal fails to meet the moment,” Mr. Roy said. “It does not meaningfully change spending. Plus, many of the decent provisions and cuts don’t begin until 2029 and beyond. That is swamp accounting to dodge real savings.”

As the day began on Thursday, Mr. Roy was alone in declaring his intention to tank the bill in committee — a declaration that came roughly half an hour after the Texas Republican entered a meeting in Speaker Mike Johnson’s office meant to assuage holdout lawmakers.

But by the afternoon, a growing number of Republicans on the panel were venting their frustrations with the measure, and it appeared that House G.O.P. leaders lacked the support, for now, to push the legislation carrying Mr. Trump’s domestic agenda over its next hurdle.