The number of Iraqi battalions capable of combat without U.S. support has dropped from three to one, the top American commander in Iraq told Congress Thursday, prompting Republicans to question whether U.S. troops will be able to withdraw next year.

Gen. George Casey (search), softening his previous comments that a "fairly substantial" pull out could begin next spring and summer, told lawmakers that troops could begin coming home from Iraq next year depending on conditions during and after the upcoming elections there.

"The next 75 days are going to be critical for what happens," Casey told the Senate Armed Services Committee.

The Bush administration says training Iraqi security forces to defend their own country is the key to bringing home U.S. troops. But Republicans pressed Casey on whether the United States was backsliding in its efforts to train Iraqis.

In June, the Pentagon told lawmakers that three Iraqi battalions were fully trained, equipped and capable of operating independently. On Thursday, Casey said only one battalion is ready.

"It doesn't feel like progress," said Sen. Susan Collins (search), R-Maine.

Despite the drop, Casey hailed significant progress in training Iraqi security forces and noted that U.S. troops are embedded with more Iraqi units in mentoring roles than before. "Have we lost ground? Absolutely not," Casey said.

Casey said the Pentagon's standard for what constitutes a fully capable Iraqi battalion is high and that it's been difficult to ensure logistical support for Iraqi units. "I understand how it could be perceived as disappointing," he told Collins.

Sen. John McCain (search), R-Ariz., questioned why the generals are discussing troop withdrawals when it's clear Iraqi security forces aren't ready.

"You're taking a very big gamble here. I hope you're correct. I don't see the indicators yet that we are ready to plan or begin troop withdrawals given the overall security situation. And that just isn't my opinion alone," he said.

Casey, the most senior commander of coalition forces in Iraq, said the result of the upcoming Iraqi referendum on a new constitution on Oct. 15 and December elections will affect whether conditions on the ground will be appropriate for withdrawing U.S. troops.

By the December elections, Casey said, the number of Iraqi security forces available will rise to 100,000, allowing the United States to ask for only 2,000 more U.S. troops compared with the 12,000 extra needed during last January's elections.

He and Gen. John Abizaid, the commander of U.S. Central Command, were testifying before the Senate and House Armed Services committees alongside Defense Secretary Donald H. Rumsfeld and Gen. Richard B. Myers, the outgoing chairman of the Joint Chiefs of Staff.

The hearing came on a day when five American soldiers were killed by a roadside bomb in Ramadi in western Iraq. That brought the number of U.S. troops who have died in Iraq since the war began in 2003 to 1,934, according to a tally by the Associated Press.

President Bush sent the group to Capitol Hill for back-to-back House and Senate hearings to try to convince lawmakers — and their skeptical constituents — that the United States is making progress in the war despite slipping support at home.

Before the Senate panel, the generals warned that defeating the insurgency would take time. "To be sure the next couple of months are going to be tough," Casey said, warning that the insurgents will ramp up their efforts in the coming weeks to try to affect the political process.

While the Bush administration has refused to set a timetable for the withdrawal of U.S. troops, Casey has repeatedly said a "fairly substantial" pullout could begin next spring and summer as long as the political process stayed on track, the insurgency did not expand and the training of Iraqi security forces continued as planned.

But when reporters asked Casey on Wednesday whether he still believed that to be the case, he said, "I think right now we're in a period of a little greater uncertainty than when I was asked that question back in July and March."

"Until we're done with this political process here with the referendum and the elections in December, I think it's too soon to tell," Casey said.

Congress has not held a hearing on Iraq with top officials since before its August break.

Back home in their districts, lawmakers heard from constituents whose discomfort about Iraq was reflected in polls showing sliding support for Bush and the war effort.

Republicans have increasingly started expressing concern, although most continue to support the president, while Democrats have begun to intensify their criticism of Bush's Iraq policy as the U.S. death toll approaches 2,000.