A House vote to label the century-old deaths of Armenians as genocide was in jeopardy Tuesday after several Democrats withdrew their support and sounded alarms it could cripple U.S. relations with Turkey.

The loss of support is a major setback to House Speaker Nancy Pelosi and other Democratic leaders on Capitol Hill, who have fiercely defended the resolution to Republicans and the Bush administration as a moral imperative in condemning the World War I-era killings of up to 1.5 million Armenians by Ottoman Turks.

President Bush called Pelosi on Tuesday to ask her not to call for a House vote on the resolution.

"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," Pelosi spokesman Nadeam Elshami said, noting that Bush initiated the phone call.

House Majority Leader Steny Hoyer, D-Md., told reporters Tuesday that the plan remained to vote on the measure before Congress adjourns by the end of the year.

But, he added, "there are a number of people who are revisiting their own positions and we'll have to determine where everyone is," he said.

The most notable Democratic challenge mounted this week came from Rep. John Murtha, an anti-war ally of Pelosi, D-Calif., and chairman of the House Defense Appropriations Subcommittee. Murtha fought against a similar measure 20 years ago.

"From my discussions with our military commanders and foreign policy experts, I believe that this resolution could harm our relations with Turkey and therefore our strategic interests in the region," Murtha, D-Pa., said in an e-mailed statement on Tuesday.

Also this week, at least six Democrats withdrew their sponsorship of the bill and two other Democrats, Reps. Alcee Hastings of Florida and John Tanner of Tennessee, asked Pelosi to forgo the vote.

Hastings, who has voted against combat funding for Iraq, and Tanner, a member of a conservative Democratic coalition known as the Blue Dogs, said they feared backlash from Turkey would cut off U.S. access to a critical air base.

"More than half of the cargo flown into Iraq and Afghanistan comes through Incirlik Air Base and this base would be a key component of any plans for redeployment of our troops in the future," the lawmakers wrote.

Rep. Ike Skelton, D-Mo., chairman of the House Armed Services Committee, and Rep. Solomon Ortiz, D-Tex., sent Pelosi a similar letter last week.

In response to last week's approval of the resolution by the House Foreign Affairs Committee, Turkey recalled its ambassador in Washington back to Ankara for consultations and asked the Bush administration to stop the resolution from passing in a final floor vote.

Turkey denies the deaths constituted genocide, saying that the toll has been inflated and that those killed were victims of civil war and unrest.

At the Pentagon, Lt. Gen. Carter Ham, operations chief for the Joint Chiefs of Staff, was asked whether the U.S. military was considering providing assistance to Turkey in the event that it went after the rebel Kurdistan Workers' Party, or PKK, in northern Iraq.

"It would be inappropriate for me from this standpoint to say we are ruling out or ruling in specific military options," Ham said, adding that Gen. David Petraeus, the top U.S. commander in Iraq, and other commanders are watching the situation closely.

"I don't know specifically what they are planning to do if this matter continues to evolve," Ham said.

He also said that because there is only one principal overland route from Turkey into northern Iraq for transporting military supplies, a Turkish land offensive southward would have "a significant effect" on the U.S. military's ability to resupply its forces in northern Iraq.