John Boehner (Chip Somodevilla/Getty Images)
Speaking to reporters at his weekly press conference, Boehner, who had previously said he wouldn't endorse a candidate, said he was "proud" to back the former Massachusetts governor.
"I've not taken a position on this presidential campaign because, as chairman of the convention, I wanted to make sure all candidates had a fair process and a fair opportunity," Boehner said. "But it's clear now Mitt Romney is going to be our nominee."
The GOP leader said he would "do everything" to help Romney win the general election against President Barack Obama.
Boehner joins several other high-profile House Republicans, including his deputy, Majority Leader Eric Cantor, who have backed Romney in recent weeks.
The speaker's endorsement is yet more bad news for Newt Gingrich, who has pledged to stay in the race until Romney wins the 1,144 delegates necessary to officially clinch the GOP nomination.
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