Romney had already received the support of Bush's parents, ex-President George H.W. Bush and Barbara Bush, back in March, with an official announcement.
I do think it's time for the party to get behind Gov. Romney, the elder Bush said then.
Barbara and I are very proud to fully and enthusiastically endorse and support our old friend Mitt Romney, he added. He's a good man, he'll make a great president.
George W.'s brother, former Florida governor Jeb Bush, has endorsed Romney too.
News of George W.'s support for Romney came a day after Republican presidential candidate Ron Paul said he won't campaign in future primaries.
Though Paul conceded he can't win the presidential nomination, his campaign told the Washington Post that Paul isn't dropping out of the race.
Unfortunately, barring something very unforeseen, our delegate total will not be strong enough to win the nomination, campaign chairman Jesse Benton wrote in a strategy memo. However, our delegates can still make a major impact at the national convention and beyond.