The Bush administration and top American generals have been vocal in warning that passage of the resolution could cause great harm to the American war effort in Iraq and have put significant pressure on Republicans to abandon their support for the measure. President Bush called Ms. Pelosi on Tuesday and asked her to prevent a floor vote.

“The president and the speaker exchanged candid views on the subject and the speaker explained the strong bipartisan support in the House for the resolution,” said Brendan Daly, a spokesman for Ms. Pelosi.

The Democratic leadership was examining the exact level of that support to gauge its next step, but lawmakers and officials said it was now unclear whether the resolution could be approved, given Republican resistance and Democratic defections. “We will have to determine where everyone is,” said Representative Steny H. Hoyer of Maryland, the majority leader.

Ms. Pelosi, who has promised a vote on the resolution if it cleared the committee, said she was leaving it to its chief backers to round up votes. “I have never known a count,” she said.

Backers of the resolution, which has the fervent support of the Armenian-American community, described the shift as slight and attributed it to the intense lobbying by the Turkish government, the administration and their allies. They said they would try to change the minds of some of those who were wavering.

“This is what happens when you are up against a very sophisticated multimillion-dollar campaign,” said Representative Brad Sherman, Democrat of California, who chided the Turkish government. “Since when has it become fashionable for friends to threaten friends?”

But he acknowledged there was little margin of error for backers of the resolution, which had once boasted 225 co-sponsors. “If the vote were held today, I would not want to bet my house on the outcome,” he said.