WASHINGTON — A year from now, the pressing question for presidential candidates in both parties may not be, “Were you for the war?” but rather, “Were you for the surge?”

Before and after President Bush’s speech to the nation Wednesday night, hopefuls to replace him had come out strongly on either side of his decision to send more than 20,000 additional troops to Iraq to try to stem the violence in Baghdad. The decisions could help or haunt them in 2008, when the country should have a clearer picture of whether the move succeeds in stabilizing the region.

For the most part, the reactions fell along party lines. On the Republican side, Senator McCain of Arizona, Governor Romney of Massachusetts, and Mayor Giuliani all voiced support for Mr. Bush’s plan. The lone exceptions were Senator Brownback of Kansas, who is traveling in Iraq and issued a statement from Baghdad opposing an increase in troop levels, and Senator Hagel of Nebraska, who criticized the shift during a Senate Foreign Relations Committee hearing yesterday,

The likely Democratic candidates were unanimous in their condemnation of what they deemed an escalation of the war. Senator Obama of Illinois called the president’s policy “wrongheaded” and said on NBC’s “Today” show yesterday that he would look to “constrain” Mr. Bush to ensure that benchmarks are achieved in Iraq. Senator Clinton of New York issued a statement shortly after the speech, saying Mr. Bush was taking the “wrong road,” but she stopped short of backing congressional action to inhibit Mr. Bush.

Senator Dodd of Connecticut, who yesterday announced his candidacy for the White House, and Senator Kerry of Massachusetts, who may yet run a second time, were among Democrats who harshly criticized Mr. Bush, as did Senators Edwards of North Carolina and Biden of Delaware.

The candidate with the most to win or lose could be Mr. McCain, who called for sending more forces even before the White House began considering the option seriously. An early front-runner for the GOP nomination, Mr. McCain said on CNN’s “Larry King Live” Wednesday night that the president’s speech was “excellent,” but he added there was no guarantee of success. He appeared to acknowledge, however, that he and Mr. Bush were linked to the outcome in Iraq. “I don’t think there’s any doubt that President Bush’s presidency … will be judged on success or failure. My time in the Senate may also be judged to some degree by it,” he said.

For Messrs. Giuliani and Romney, considered the top Republican rivals to Mr. McCain, the calculation to take a position on the troop increase was more complicated than for the Arizona senator, a decorated Vietnam veteran who is considered a leading voice on foreign policy.

Both men issued statements prior to Mr. Bush’s speech, and Mr. Giuliani appeared on ABC’s “Good Morning America” yesterday to discuss Iraq and his possible candidacy. Neither the former mayor nor Mr. Romney have military backgrounds, and Mr. Giuliani said on ABC that he spent the last week or two “being briefed by everybody who would brief me, generals and others, on this.” He cited his time as mayor, comparing the difficulty of keeping Baghdad secure to battling high-crime areas of New York.

A Romney spokesman, Kevin Madden, said the former Massachusetts governor had also come to his decision after consultation with military experts and retired generals. “This was not a political consideration,” Mr. Madden said.

With the outcome of Mr. Bush’s troop increase far from assured, a Republican strategist based in Washington, Rich Galen, said he would have advised Messrs. Romney and Giuliani against backing the plan outright and instead counseled them to offer the president more general support and hopes for success. But they were in a difficult position, he said. “Part of running for president is that you’ve got to take stands when you wish you didn’t have to,” he said.

Another GOP consultant, Curt Anderson, said that despite the defections of Messrs. Brownback and Hagel, opposing the president wouldn’t be “politically tenable” for a hopeful who wanted to be considered a top contender for the Republican nomination.

As Republicans and Democrats took to the airwaves to debate Iraq, a long shot for 2008, Governor Vilsack of Iowa, announced yesterday that he would step down as the chairman of the centrist Democratic Leadership Council to concentrate on his bid for the White House. Mr. Vilsack has led that group for 18 months and is looking to follow the path of President Clinton, who used his position as chairman to launch his 1992 bid.