Former President George Bush Senior has endorsed John McCain in his bid to be the Republicans' presidential nominee.
George Bush Senior praised Mr McCain's character and values
Mr Bush, father of the current US president, said Mr McCain's character was "forged in the crucible of war" and he was best prepared to lead the US.
The endorsement, which may help unite the party behind Mr McCain, comes ahead of bi-party primary votes in Wisconsin and a Democratic primary in Hawaii.
Mr McCain is leading the Republican race, ahead of Mike Huckabee.
He is considered almost certain to be the eventual Republican nominee, having already won 843 of the 1,191 delegates he needs to secure the nomination.
However, Mr McCain has not been popular with more conservative Republicans because of his relatively moderate views on abortion, immigration reform and gay marriage.
'Character and values'
Speaking in Texas with Mr McCain by his side, Mr Bush Senior said he believed Mr McCain was the best equipped to lead the country.
"No-one is better prepared to lead our nation at these trying times than Senator John McCain," Mr Bush said.
"His character was forged in the crucible of war. His commitment to America is beyond any doubt. But most importantly, he has the right character and values to guide our nation."
Mr Bush dismissed criticism of Mr McCain's conservative credentials by some high-profile commentators as "grossly unfair".
He said Mr McCain had "a sound conservative record" and also praised his ability to reach across the aisle to the Democrats when needed.
Mr McCain's failed effort to forge bi-partisan legislation to reform the US immigration system last year has troubled many in the party's more conservative wing.
Mr McCain, a Vietnam veteran who spent more than five years as a prisoner of war, said he was honoured to have the support of the former president.
"I think that our effort to continue to unite the party will be enhanced dramatically by President Bush's words," he said.
Mr Bush's endorsement comes only four days after that of Mitt Romney, the former contender for the nomination who dropped out of the race after disappointing results earlier this month.
Meanwhile, Hillary Clinton and Barack Obama have been campaigning for the Democratic Party nomination in Wisconsin over the weekend.
Mr Obama and Mrs Clinton attended a Wisconsin Democratic party event
Mrs Clinton released a detailed economic plan on Monday, which her campaign hopes will win her support among working and middle class families struggling amid the country's economic downturn.
Polls suggest Tuesday's vote in Wisconsin will be close.
Wisconsin has 92 delegates up for grabs, while Hawaii - where Mr Obama was born - has 20 on offer.
Mr Obama's campaign confirmed that he had travelled to North Carolina on Sunday for a meeting with former Democratic contender John Edwards, who suspended his campaign before the 5 February Super Tuesday vote.
He told a Wisconsin TV channel that the meeting had been "to talk about how we can move the party in a direction that focuses on middle-class issues - relieving poverty, reducing the influence of special interests in Washington".
Both Mr Obama and Mrs Clinton are also focusing their efforts on the delegate-rich states of Texas and Ohio, which will stage primary elections on 4 March.
Mrs Clinton is currently trailing Barack Obama with 1,220 delegates to his 1,275. It will take 2,025 delegates to secure the nomination at the party's national convention this summer.