"No one is better prepared to lead our nation at these trying times than Sen. John McCain," Mr. Bush said, standing alongside the Republican nominee-in-waiting in an airport hanger. "His character was forged in the crucible of war. His commitment to America is beyond any doubt. But most importantly, he has the right values and experience to guide our nation forward at this historic moment."
McCain, in turn, said he was deeply honored by Mr. Bush's support. "I think that our effort to continue to unite the party will be enhanced dramatically by President Bush's words," he added.
Since effectively sealing the nomination when chief rival Mitt Romney dropped out, McCain has been working to convince the fickle and influential conservative base of the Republican Party to get behind his candidacy.
He's seen some progress, with several high profile Republicans from the party's establishment endorsing McCain in an effort to unite the party while Democrats continue to fight for a nominee. Still, McCain has much work to do to energize the party behind his candidacy to ensure that its people turn out this fall.
President Bush has spoken warmly of McCain, calling him a "true conservative." But he also has said that McCain might have to work harder to win over the support of the GOP's more conservative wing. Protocol demands that he not swing explicitly behind the candidate with a race still technically - and only technically - in progress.
His father's endorsement, which follows one from former Florida Gov. Jeb Bush, who is George W. Bush's brother, is a further nudge by GOP chieftains for conservative activists to get over their distaste for McCain and for rival to get out.
Without mentioning McCain's chief standing rival by name, the elder Bush suggested that he wasn't sending a signal to Huckabee, the former Arkansas governor. "I had not come here to tell any other candidate what to do," Mr. Bush said.
Still, he recalled his own defeat in the 1980 presidential race, and said: "It can take a while for any candidate to read the handwriting on the wall, and that certainly was true of me."
Mr. Bush also called criticism by the right flank that McCain is not conservative enough absurd and grossly unfair.
"He's got ... a sound conservative record, and yet he's not above reaching out to the other side," Mr. Bush said.
McCain has drawn the ire of some high-profile conservative pundits and others for what they call infractions against the party. McCain twice voted against Bush's tax cuts. He pushed a campaign finance overhaul that critics said restricted their free speech rights. And, he has worked across the aisle with Democrats on issues like an eventual path to citizenship for millions of illegal immigrants - heresy in the eyes of many hard-core Republicans.
As he makes the transition into a general election candidate, McCain not only must rally the party but also must try to determine how to deploy the current president, whose job approval rating is at a low point.
While still popular among Republicans, many moderates and independents have turned from the president, and Democrats already have started casting McCain's candidacy as a continuation of Bush's eight years in office.
But McCain shows little willingness to distance himself, saying: "I'd be honored to have President George Bush's support, his endorsement. And I'd be honored to be anywhere with him under any circumstances."
However, he added: "Obviously, ... any president who follows one has different views on particularly specific issues."