Washington: Former US president George H.W. Bush Monday endorsed Republican candidate John McCain, calling him the best person to lead the US at a "critical time in history".
Bush said the Arizona senator was a "remarkable patriot" whose sacrifice for the country was unparalleled.
McCain, 71, a prisoner of war in Vietnam, has staked his campaign on success in Iraq and is virtually guaranteed of winning the Republican presidential nomination.
"At this critical time in history ... no one is better prepared to lead our nation at these trying times than Senator John McCain," Bush said at a joint event with McCain in Houston.
"Part of our Republican creed is a prevailing sense of duty, and in the coming election we do not have the luxury of taking a pass on our unique role and responsibilities in the world," he said.
The former president also offered some advice to McCain's one remaining Republican opponent, Mike Huckabee, saying that he, too, had a hard time realising his presidential campaign was doomed when he ran in 1980.
"After so much time and exhaustive effort by so many friends, it can take a while for any candidate to read the handwriting on the wall, and that certainly was true of me," Bush said.
Bush ran against Ronald Reagan for the Republican nomination in 1980. He was elected president in 1988 after eight years as Reagan's vice president.
Huckabee lags far behind McCain in the battle for delegates to the Republicans' nominating convention in September but has refused to drop out of the race.
Even as the presumptive nominee, McCain has drawn the ire of some conservatives for his maverick reputation in the party and moderate views on a number of domestic issues.
But Bush, father of current President George W. Bush, said it was time for the Republican party to unite in order to take on the Democrats in the Nov 4 general election.
"I believe now is the right time for me to help John in his effort to start building the broad-based coalition it'll take for our values to carry the White House this fall," he said.