WASHINGTON (CNN) -- President Bush endorsed Sen. John McCain for president on Wednesday, saying the presumptive Republican nominee has the "character, courage and perseverance" to lead the country.
McCain thanked the president for his support and the work he has done in the Oval Office.
"I appreciate his endorsement, and I appreciate his service to our country," said McCain, adding that he wanted Bush at his side as much as possible on the campaign trail.
"Whatever he wants me to do, I want him to win," Bush said, who was challenged by McCain for the GOP nomination in 2000. But he said the 2008 run for the Oval Office was not his battle.
"It's not about me. I've done my bit," Bush said.
Addressing the calls for change in the presidential campaign, Bush said McCain would be steadfast to one of his administration's policies.
"He's not gonna change when it comes to taking on the enemy," Bush said of the senator from Arizona. Watch Bush explain why McCain should be the next president »
Protecting the American people was the No. 1 job of a president and McCain understood that, Bush said.
"He's gonna be a president who will bring determination to defeat an enemy," Bush said.
McCain clinched the GOP presidential nomination with victories Tuesday in Ohio, Texas, Vermont and Rhode Island. Watch McCain say how he will prevail in the fall
McCain needed 1,191 delegates to secure the nomination and had 1,226 after Tuesday's voting, according to CNN estimates.
McCain said with the nomination secured, he would begin exploring possible running mates.
He also said he called both Sens. Barack Obama and Hillary Clinton and pledged "a respectful campaign" no matter who wins the Democratic nomination.
Before Wednesday's event at the White House, both Republicans and their Democratic opponents expressed excitement about the possibilities of Bush endorsing McCain.
Republican Sen. Kay Bailey Hutchison of Texas said McCain can now focus on solidifying support among conservative Republicans, the majority of whom backed candidates other than the Arizona lawmaker in the primaries, according to exit polls.
"I think the endorsement of President Bush will certainly go a long ways toward that," Hutchison said Wednesday. "John McCain is going to be very focused on our base and the people that he wants to have in full force behind him." Watch Bush greet McCain at the White House »
Despite overall approval ratings hovering just above 30 percent, Bush receives far higher marks from conservatives, and the McCain campaign thinks the push from Bush will bring the party in line behind their presumptive nominee.
"He'll have the [Republican National Committee] behind him. He'll have a broad base of financial support. It's a big step," said Alex Castellanos, a GOP strategist and CNN contributor.
A senior administration official concurred Wednesday, saying Bush will raise "a lot of money" for McCain.
"He is extremely popular" with the GOP base, the official said. "And so can do a lot to drive the base in the election, which will help across the board."
William Bennett, a CNN contributor who was in the administrations of Presidents George H.W. Bush and Reagan, said Clinton and Obama will have to factor McCain into their strategies to secure the Democratic nomination, something that should help the Republican define whomever becomes his November foe.
"They have to factor that in as they debate each other every time they put out an ad and make a position," Bennett said.
But Democratic strategist and CNN contributor Paul Begala said his party is happy to see McCain get the nomination -- and happy to see anything the senator does that links him more closely with the Republican president.
"He's embraced the Bush tax cuts that he voted against. He was against them being temporary; now he wants them being permanent. That's like marrying a girl you didn't want to date. He rushed to Bush's Social Security plan, even disavowing his own Social Security plan on his own Web site. He has now become Bush's third term," Begala said.
Begala called McCain's victory speech Tuesday night "an eloquent but not very energetic defense of the status quo." Watch McCain speak to supporters after clinching the nomination »
"Democrats heard that speech and loved it," he said. "To quote our current president, bring it on." E-mail to a friend