Former Republican US presidential nominee Mitt Romney will not attend the party's convention in July, joining other former Republican Oval Office candidates and inhabitants in spurning Donald Trump's candidacy, according to a newspaper report Thursday.

Romney, who lost to Barack Obama in the 2012 race, will skip the convention in Cleveland, Ohio, which appears set to officially name Trump as the party's nominee in the November general election, the Washington Post reported, citing a Romney aide.

The former Massachusetts governor's decision comes a day after Trump's last competitor in the 2016 Republican contest, Ohio Governor John Kasich, suspended his campaign, leaving Trump with no opposition in the race to become the party's nominee. Ted Cruz, a US senator from Texas, suspended his campaign on Tuesday.

Romney has been an outspoken critic of the New York billionaire. In a speech in March, he called Trump "a phony and a fraud" whose candidacy would have "profound consequences" for the nation and the world.

Romney's decision to skip the convention follows a similar choice by 2008 Republican candidate John McCain, a US senator from Arizona who was an early target of a Trump insult.

On Wednesday, the Texas Tribune reported that former Republican presidents George W and George HW Bush had no plans to endorse Trump or attend the party's nominating event.

Jeb Bush, the brother and son of the former presidents, was considered the Republican favourite going into the 2016 race and held dominant fundraising edge. However, he was subject to a slew of Trump put-downs, and eventually left the race after his candidacy failed to gain traction.

Speaking to broadcaster CNBC on Thursday, Trump was unfazed by the Bushes' decision not to attend the convention.

"I think that's fine," he said, noting that throughout the campaign he'd been hard on Jeb Bush and highly critical of George W Bush's decision as president to invade Iraq. "I don't care if they sit it out."

Speaker of the House Paul Ryan, who has repeatedly sought to distance himself from the front-runner, said Thursday he wasn't prepared to throw his support behind Trump.

"I'm just not ready to do that at this point. I'm not there right now," the top Republican in Congress told CNN.

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