WASHINGTON — President Bush endorsed Republican nominee-in-waiting Senator McCain today, two bitter rivals from the 2000 presidential race joining together now in hopes of preventing Democrats from winning the White House this fall.

“John showed incredible courage, strength of character, and perseverance in order to get to this moment and that’s exactly what we need in a president — somebody who can handle the tough decisions, somebody who won’t flinch in the face of danger,” Mr. Bush said, appearing with Mr. McCain in the Rose Garden.

Mr. Bush’s embrace of the Arizona senator as the party’s next standard-bearer comes a day after Mr. McCain clinched the Republican Party nomination by getting the requisite 1,191 convention delegates. Republicans won’t officially nominate Mr. McCain until early September at the Republican Party’s national convention in Minneapolis-St. Paul.

“A while back I don’t think many people would have thought that John McCain would be here as the nominee of the Republican Party,” Mr. Bush said. “Except he knew he’d be here and so did his wife, Cindy.”

With his low poll ratings and an unpopular war on his shoulders, Mr. Bush could hurt Mr. McCain with some groups, while helping with others.

“If my showing up and endorsing him helps him — or if I’m against him and it helps him — either way, I want him to win,” the president said. “This is an age-old question that every president has had to answer, and there is an appropriate amount of campaigning for me to do. But they’re not going to be voting for me.”

“They’re not going to be voting for me,” the president said. “I’ve had my time in the Oval Office.”

“It’s not about me,” Mr. Bush said. “I’ve done my bit.”

Mr. Bush gave Mr. McCain a big welcome at the White House, greeting him at the North Portico entrance usually reserved for visiting foreign dignitaries.

Mr. McCain showed up late and kept Mr. Bush waiting. The president whiled away the time by dancing and joking and laughing for the cameras.

Mr. McCain said he looked forward to campaigning with Mr. Bush at his side and said the president could be helpful in states such as Texas. Mr. Bush pledged to do whatever he could — even getting out of the way when that would help.

“I got a lot to do, but I’m going to find ample time to help,” Mr. Bush said. “I can help raise him money, and if he wants my pretty face standing by his side at one of these rallies, I’d be glad to show up.”

“But they’re going to be looking at him. I’m going to be in Crawford with my feet up,” Mr. Bush said. “He’s going to be sitting in behind that desk making decisions on war and peace.”

Mr. McCain said he had great respect and admiration for Mr. Bush.

“I intend to have as much possible campaigning events together as is in keeping with the president’s heavy schedule,” Mr. McCain said.

“I hope that the president will find time from his busy schedule to be out of the campaign trail with me, and I will be very privileged to have the opportunity of being again on the campaign trail with him.”

Mr. McCain’s Washington visit amounted to a victory lap of sorts after a bruising 16-month Republican presidential primary. He was visiting not only the White House he hopes to occupy but also the Republican National Committee headquarters that he essentially assumes control of now that he’s the expected Republican nominee.

He was essentially laying claim to the entire force of the Republican Party apparatus as he plots his general election strategy and sets in motion his campaign — and that of the party — to keep a Republican at the White House helm.

For Mr. McCain, the general election campaign starts now even though Democrats still haven’t chosen a candidate. Senators Obama and Clinton continue a protracted battle for their party’s nod, leaving Mr. McCain an opportunity to unify his party.

To that end, Mr. Bush’s support sends a strong signal to Republican critics of Mr. McCain to fall in line.

The Republican Party’s conservative base has resisted rallying around Mr. McCain, long viewing him skeptically for working across the aisle with Democrats on issues that the right flank detest.

Mr. Bush is the head of the Republican Party and he remains a well-liked figure with the Republican rank-and-file. Thus, he could be an asset in raising money and rallying the Republican base for Mr. McCain.

However, his job performance rating is at a low point and he is unpopular with the general public.