Sensing the inevitability of a Romney victory in the battle to see which Republican squares off against President Barack Obama, House Speaker John Boehner ended his neutrality, saying he'd be "proud" to help him win the November election.
"It's clear now Mitt Romney is going to be our nominee," Boehner told reporters following a bruising, months-long nominations race.
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"And I will be proud to support Mitt Romney and do everything I can to help him win."
Senate Minority Leader Mitch McConnell followed suit a few hours later.
"I support governor Romney for president of the United States," McConnell told reporters.
"He's going to be the nominee," he said, noting how the party "is in the process of unifying behind him."
Two weeks ago, McConnell described Romney as an "outstanding" nominee who could prevent an Obama re-election.
But he had stopped short of a direct endorsement at the time, arguing that people in Wisconsin, Maryland and the US federal capital Washington, who were voting that week, did not need his help in choosing their candidate.
Romney won all three of those contests, and with his main challenger Rick Santorum dropping out of the race, it became clear that Romney was going to be the nominee.
"We're all behind him, and looking forward to the fall campaign which is actually under way," McConnell said Tuesday.
"I think it's going to be an incredibly close and hard-fought race."
Democrat Harry Reid, the Senate majority leader who spoke minutes after McConnell, seemed to revel in the campaign ahead as he made light of McConnell's endorsement, quipping to reporters: "I'm supporting the winner of the next election in November -- Barack Obama!"
The president is ahead in the personal popularity stakes, but a new poll taken after Santorum dropped out shows Romney making important gains, particularly among conservative voters.
Some 69 per cent of Republicans, including 80 per cent of conservative Republicans, now hold favourable views of Romney, both career highs, according to the Washington Post/ABC News poll.
Obama's popularity has risen as well, matching a two-year high, with women holding especially favourable views of the president, 58-36 per cent, according to the Post.
Romney trails badly among women, with nearly twice as many expressing unfavourable (52 per cent) as favourable views (27 per cent).
Several leading party lawmakers, including House majority leader Eric Cantor and Senator Marco Rubio of Florida, had thrown their support behind Romney, a former governor of Massachusetts, weeks ago.
But distinctly absent in Romney's endorsement column is Santorum, and that could prove difficult down the road for Romney as he seeks to win over sceptics such as evangelical voters, who supported Santorum more than Romney in the primaries.
The deeply religious ex-senator from Pennsylvania made no mention of Romney in his speech announcing his capitulation, leading to speculation there was bad blood between the rivals, who frequently clashed during the campaign.
Although former House speaker Newt Gingrich and Libertarian-leaning congressman Ron Paul remain in the Republican race, neither has a real chance of winning.
Romney has this month switched his focus to attacking Obama rather than other Republican contenders, a move that has become more pronounced since Santorum's exit.
He campaigned in Santorum's home state of Pennsylvania and earned the endorsement of Governor Tom Corbett, who said Romney "will restore fiscal sanity to Washington by cutting spending, lowering taxes, and reforming entitlements."
On the day of the US tax-filing deadline, Romney, a multi-millionaire former venture capitalist, was in the Pittsburgh suburb of Bethel Park, where he sat down with families around a picnic table filled with lemonade and pretzels to discuss his plan to "lower the tax rates."
He said he was also seeking to "limit some of the deductions and exemptions and breaks, particularly for high-income folks," to bring in sufficient revenue.
But he hailed the Senate's defeat of the Obama-backed "Buffett Rule," calling the effort to raise the tax rates for millionaires and billionaires "more of a gimmick than... a real proposal to reform our tax system.”
