Former Republican presidential candidate Mitt Romney has endorsed John McCain's White House bid.
Speaking alongside Mr McCain in Boston, the former Massachusetts governor said he was honoured to give his full support to "a true American hero".
Mr Romney had attracted strong conservative support before he dropped out of the race for the White House nomination last week.
Mr McCain is now widely expected to be the eventual Republican nominee.
Despite their previous disagreements over policy issues, Mr Romney told the joint press conference there was "no doubt in my mind that this individual should be the next President of the United States".
"I am officially endorsing his candidacy and today I'm asking my delegates to vote for Senator McCain at the convention.
"This is a man capable of leading our country at a dangerous hour. Senator McCain understands the war we're in, the necessity of victory, and the consequences of surrender.
"For him national security is not just another agenda item, it is the abiding concern and responsibility of the nation."
Mr McCain said it was now time to "move forward together for the good of our party and our nation".
The candidates have often strongly disagreed during debates
Mr McCain is leading the contest for nomination as the Republican presidential candidate, ahead of Mike Huckabee and Ron Paul.
The BBC's North America editor Justin Webb says Mr McCain will almost certainly 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 civil marriages.
Mr McCain brushed aside questions from the media regarding his disagreements with Mr Romney, saying that they shared a "common philosophy".
"We had differences on specific issues, but there was never any doubt about the common philosophy and principles and dedication to the party of Abraham Lincoln, Theodore Roosevelt and Ronald Reagan that we share," he said.
Mr Romney withdrew his candidacy last week saying that if he continued to fight he would "forestall the launch of a national campaign and make it more likely that [Democratic candidates] Senator Clinton or Obama would win".
His withdrawal was a disappointment for conservatives who now have to decide whether to switch their allegiance to Mr McCain or to the other remaining Republican candidates.
After today's announcement, Mr Huckabee said the elections "should be about choices and voices, and not a coronation", reported Reuters.
"Until someone receives 1,191 delegates, the verdict is not in."
Meanwhile, Hillary Clinton has been campaigning for the Democratic Party nomination in Ohio, which will stage a primary election on 4 March.
Mrs Clinton is trailing Barack Obama with 1,220 delegates to his 1,275.