WASHINGTON – When he takes the stage tonight to call on the country to re-elect President Bush, Senator McCain of Arizona will offer his once bitter rival a truce, an endorsement, and a bridge to swing voters.

In return, he is receiving a national platform, an embrace back into the fold of the Republican Party, and perhaps most surprisingly, full-throated presidential backing of his ongoing campaign to clamp down on money in politics.

It is a barter of mutual advantage that would have been unimaginable in 2000, when the two battled for the party nomination in a bitter contest. Back then Mr. McCain railed against the “sleaze” of attack ads run by Mr. Bush’s allies.

Even a few months ago, speculation abounded that Mr. McCain could run against the president on the Democratic ticket with his friend and fellow Vietnam War veteran, Senator Kerry. Though he denied the rumors, and requested that Mr. Kerry take down television ads using his name, he was eyed with suspicion in many Republican circles.

Mr. McCain has now “buried the hatchet” from 2000, and has observed that, “Americans don’t like a sore loser.” In return, he has been tapped to offer his personal testimonial to the president’s leadership before the party and the nation.

“As with all relationships, there is always an evolution,” said a Bush campaign spokesman, Kevin Madden. The once tense McCain-Bush relationship is now one of “mutual cooperation and respect,” he said.

“It’s not like the two are ever going to go on weekend camping trips together,” said a former McCain communications director, Dan Schnur. “But it’s clear they’ve found a whole issue array in which they are in agreement.”

The payoff for Mr. McCain’s faithful soldiering – campaigning aggressively for the president across the country – has turned out to be perhaps greater than he could have expected.

Last week, the president surprised many observers by not only endorsing Mr. McCain’s efforts to rein in independent groups that run political ads funded by unlimited contributions, but also expressing the view that their activities should be banned altogether.

While it may have been an effort by Mr. Bush to distance himself from attack ads against Mr. Kerry by the Swift Boat Veterans for Truth group without denouncing their ads directly, it has bolstered Mr. McCain’s cause. Mr. Bush has thrown the weight of the presidency behind strict enforcement of Mr. McCain’s signature achievement, the McCain-Feingold campaign finance laws, which had been signed by Mr. Bush in March 2002 without fanfare and condemned by many prominent Republicans as unconstitutional.

“It was the new association with McCain that almost forced him into embracing the proposal,” said a scholar at the American Enterprise Institute and founder of the institute’s Campaign Finance Reform Working Group, Norman Ornstein.

When the president said ads by 527 committees – named for the section of the tax code that allows tax-exempt groups who raise money for political activities to be organized – should be banned, the critics were silent.

“I find astonishing at this point, that I have not heard a peep – not a peep – from Mitch McConnell, Tom Delay, Dennis Hastert, or the Republicans who led the charge against McCain-Feingold as the greatest constitutional disasters. Now you have a president going even further,” said Mr. Ornstein.

The two men have “open and constant communication on the issue” of the 527 groups, said Mr. Madden. Mr. McCain’s “tremendous credibility” on the issue “helps convey the president’s seriousness,” he added.

That momentum could carry through to the rest of his campaign finance agenda. He has a bill to overhaul the Federal Election Commission that is expected to be taken up by lawmakers in the fall. His heavy legislative agenda includes everything from prescription drug importation to military and communications reform.

His role in Mr. Bush’s campaign will also help temper Mr. Mc-Cain’s maverick image ahead of a potential run for the party’s nomination in 2008.

“If he were to run again, active and visible support for the president would serve to remind primary voters that he is a loyal Republican,” said Mr. Schnur.

To Mr. Bush, Mr. McCain offers an image of tolerance and moderation by association.

His appeal among independent voters is useful to the Bush campaign that has focused heavily on mobilizing their base supporters. “He has tremendous appeal among the electorate,” marveled Mr. Madden, recalling a campaign appearance he worked with Mr. Mc-Cain in January in Nashua, N.H. The adoration the senator received from the crowds was such, said Mr. Madden, that, “I felt like a road manager for Led Zeppelin.”