President Bush's decision to deploy more than 20,000 additional troops to Iraq drew fierce opposition Thursday from congressional Democrats, but the Senate's top Republican threatened a filibuster to block any legislation expressing disapproval of the plan.

"Obviously, it will ... require 60 votes," said Sen. Mitch McConnell, R-Ky., as senior administration officials made the case for Bush's new policy in Congress, at news briefings and the morning television programs.

"This is a time for a national imperative not to fail in Iraq," Secretary of State Condoleezza Rice told the Senate Foreign Relations Committee.

On the morning after Bush's prime time speech from the White House, Senate Majority Leader Harry Reid, D-Nev., declared: "In choosing to escalate the war, the president virtually stands alone."

Many Republicans, too, were clearly tired of the war, which has cost more than 3,000 American troops their lives, and played a major role in the Democratic takeover of Congress in last fall's elections.

"At this late stage, interjecting more young American troops into the crossfire of an Iraqi civil war is simply not the right approach," said Rep. Ric Keller, R-Fla. "We are not going to solve an Iraqi political problem with an American military solution," he said in remarks on the House floor.

At a news conference, McConnell accused Democrats of secretly favoring a plan to cut off funding for the troops -- an allegation that Reid and House Speaker Nancy Pelosi, D-Calif. denied.

McConnell conceded that Republicans as well as Democrats are troubled by Bush's new policy, but said, "Congress is completely incapable of dictating the tactics of the war."

Reid has said he will schedule a vote on a nonbinding bill expressing disapproval of Bush's new policy, but McConnell's filibuster threat indicated that he would not be rushed into the vote. Under the Senate's rules, 60 votes are required to cut off debate on an issue, and even the threat of a filibuster can force concessions by the majority.