Washington (AFP) - There may be much Republican hand-wringing over Donald Trump's presumptive nomination to face the Democratic candidate for the White House, but the boastful billionaire says he doesn't care, and it doesn't matter.

A growing chorus of senior Republican leaders have joined the "anyone but Trump movement," including 2012 Republican presidential nominee Mitt Romney and the last two Republican presidents, George W. Bush and George H.W. Bush.

"Does it have to be unified?" Trump asked about the Republican Party.

"I'm very different than everybody else, perhaps, that's ever run for office. I actually don't think so," he told ABC's "This Week" in excerpts provided ahead of Sunday's broadcast.

"I think it would be better if it were unified, I think it would be -- there would be something good about it. But I don't think it actually has to be unified in the traditional sense."

A group of conservatives opposed to Trump's candidacy meanwhile announced it had launched a "formal effort" for an alternative candidate, though it stopped short of backing a contender from a third party.

"This is not just a fight for the heart and soul of the Republican Party; it is a battle for the future of our country," Conservatives Against Trump said in a statement.

"This week, Conservatives Against Trump launched a formal effort to identify an acceptable alternative candidate to run for president against Donald Trump and Hillary Clinton."

The race is still "wide open for a qualified conservative candidate," the group of activists said.

"We will not vote for Donald Trump or Hillary Clinton; but we will vote."

Trump, however, said he expected even some Democratic voters to throw their support behind him to win the general election.

"I'm going to go out and I'm going to get millions of people from the Democrats," Trump said.

"I'm going to get Bernie (Sanders) people to vote, because they like me on trade," he added, referring to the Democratic candidate in an uphill fight against Hillary Clinton to clinch the party's nomination.