WASHINGTON — Defying a fresh veto threat, the Democratic-controlled Congress will pass legislation within days requiring the withdrawal of American combat troops from Iraq beginning October 1, with a goal of completing the pullout six months later, the Senate majority leader said yesterday.

Senator Reid said the legislation “immediately transitions the U.S. military away from policing a civil war.” He said troops that remain in Iraq after next April 1 could only train Iraqi security units, protect U.S forces, and conduct “targeted counter-terror operations.”

The Nevada Democrat outlined the elements of the legislation in a speech a few hours after President Bush said he would reject any legislation along the lines of what Democrats intend to pass. “I will strongly reject an artificial timetable [for] withdrawal and/or Washington politicians trying to tell those who wear the uniform how to do their job,” the president said.

Mr. Bush made his comments to reporters in the Oval Office as he met with senior military leaders, including his top commander in Iraq, General David Petraeus.

Taken together, Mr. Reid’s speech and Mr. Bush’s comments inaugurated a week of extraordinary confrontation between the president and the new Democratic-controlled Congress over a war that has taken the lives of more than 3,200 American troops.

Negotiators for the House and Senate arranged a late-afternoon meeting to ratify the timetable that Mr. Reid laid out. The demand for a change in course will be attached to a funding bill that is needed to pay for the wars in Iraq and Afghanistan.

Under an agreement by Democratic leaders, the final bill could trigger the withdrawal of U.S. troops as early as July 1 if Mr. Bush cannot certify that the Iraqi government is making progress in disarming militias, reducing sectarian violence, and forging political compromises.

The bill also would withhold foreign aid money if the Iraqi government does not meet certain benchmarks.

As part of the same measure, congressional negotiators also tentatively agreed on about $25 billion not requested by Mr. Bush for medical care for troops and veterans, aid to the victims of Hurricane Katrina, farm disaster relief, and other programs.

The add-ons have provoked a separate veto threat from Mr. Bush. Negotiators dropped provisions ridiculed by the president such as aid to peanut farmers and spinach producers.

Mr. Reid said Mr. Bush was in “a state of denial” over the war, and likened him to another commander in chief four decades ago. “I remember when President Johnson, trying to save his political legacy, initiated the first of many surges into Vietnam in 1965,” he said.

Mr. Reid said thousands more American troops died in Vietnam in the years that followed. Now, he said, Mr. Bush “is the only person who fails to face this war’s reality — and that failure is devastating not just for Iraq’s future, but for ours.”

Mr. Reid had made similar comments at a White House meeting last week among Mr. Bush and top lawmakers, and this time, the president’s spokeswoman fired back. She said it was Mr. Reid who was ignoring reality, not the president.

Mr. Reid is in denial about the vicious nature of the enemy and about the American-led plan to provide more security in Iraq, said deputy press secretary Dana Perino. “He’s also in denial that a surrender date — he thinks it is a good idea. It is not a good idea. It is defeat. It is a death sentence for the millions of Iraqis who voted for a constitution, who voted for a government, who voted for a free and Democratic society.”

In a question and answer session after his speech, Mr. Reid was asked what the U.S. should do if U.S. troops leave and Iraq collapses into chaos.

“We know this is an intractable civil war going on now,” he responded.

Mr. Reid drew criticism from Mr. Bush and others last week when he said the war in Iraq had been lost.

The Nevada Democrat did not repeat the assertion in his prepared speech, saying, “The military mission has long since been accomplished. The failure has been political. It has been policy. It has been presidential.”

Mr. Reid said that in addition to the timetable, the legislation would establish standards for the Iraqi government to meet in terms of “making progress on security, political reconciliation and improving the lives of ordinary Iraqis who have suffered so much.”

The measure also would launch diplomatic, economic and political policy changes, the Nevada Democrat said. Mr. Reid also challenged Mr. Bush to present an alternative if, as expected, he vetoes the Democratic legislation.