Gerald Herbert, Associated Press
Former President George H.W. Bush announces his support for John McCain's candidacy with wife Barbara, Cindy McCain and John McCain behind him.

HOUSTON — John McCain picked up former President George H.W. Bush's support on Monday, a critical blessing by a pillar of the Republican establishment whose members aren't completely sold on the party's next standard-bearer.

"Few men walking among us have sacrificed so much in the cause of human freedom. And I'm happy to help this remarkable patriot carry our party banner forward," Bush said, standing alongside the GOP's nominee-in-waiting in an airport hangar.

In endorsing McCain, the patriarch of the Republican political dynasty sent a strong message to a party base wary of the Arizona senator because of his reputation for bucking the GOP on several high-profile issues. The elder Bush also signaled to a vast network of Bush family activists and fundraisers that they, too, should swing behind the eventual nominee.

McCain said he was deeply honored by Bush's support. "I think that our effort to continue to unite the party will be enhanced dramatically by President Bush's words," he said.

Since effectively sealing the nomination when chief rival Mitt Romney dropped out, McCain has been working to convince the party's fickle and influential conservative base to get behind his candidacy.

He's seen some progress, with several high-profile establishment Republicans endorsing him 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, for his part, has spoken warmly of McCain, calling him a "true conservative." But he also has said McCain might have to work harder to win over 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, the president's brother, is a further nudge by GOP chieftains for conservative activists to get over their distaste for McCain — and for rival Mike Huckabee to exit the race.

Without mentioning McCain's chief rival by name, the elder Bush suggested 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," 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."

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," 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." He added that any president who follows another in office could have different views "on particularly specific issues."

The president, during a visit to Africa, was asked Monday about reports the McCain camp wants his fundraising help but doesn't want him to appear too often with McCain.

"I'm sitting in Tanzania, I don't know what the McCain campaign said," Bush said. "But I'll help him in any way I can. Absolutely."

He went on to say, "He's (McCain) going to win, too, if he ends up being the nominee. Now, he's still got a — you know, as I understand, Mike Huckabee's still in the race. But if John's the nominee, he'll win." Bush spoke on NBC's "Today" show.