Triumphantly armed with a majority of his party's delegates, Republican Donald Trump has unleashed a broadside attack on Hillary Clinton's policies.
The New York billionaire shrugged off signs of discord in his party hours after sewing up the number of delegates needed to clinch the GOP nomination, a feat that completed an unlikely rise that has upended the political landscape and set the stage for a bitter presidential campaign.
"Here I am watching Hillary fight, and she can't close the deal," Trump crowed during an appearance in North Dakota on Thursday night.
Trump's good news was tempered by ongoing internal problems. Those include the sudden departure of his political director and continuing resistance by many Republican leaders, including House Speaker Paul Ryan and New Mexico Governor Susana Martinez, to declaring their support for his outsider candidacy.
At the same time, Clinton faced fresh questions about her use of a private email server while secretary of state, even as she fought to pivot towards Trump, who she warned would take the country "backward on every issue and value we care about".
The State Department's inspector-general released a report a day earlier concluding that Clinton did not seek legal approval for her private email server, guaranteeing the issue will continue nagging her campaign.
She insisted on Thursday that she had done nothing wrong.
Campaigning before union workers in Las Vegas, she decried Trump's anti-union comments and his proposal to deport millions of immigrants who are in the US illegally. She said he is an "unqualified loose cannon" who should never be president.
Complicating her election challenge, Clinton's Democratic rival Bernie Sanders embraced the possibility of a one-on-one debate with Trump. The Republican said he'd "love to debate Bernie", but would want the debate to raise at least $US10 million ($A13.86 million) for charity.
"The problem with debating Bernie, he's going to lose," Trump noted.
Sanders, appearing on ABC's Jimmy Kimmel Live, kept alive the notion of a debate, saying that he and Trump have "very different" views of the world. Sanders said "the goal would be to have it in some big stadium here in California".
Just 75 delegates short of her own delegate majority, Clinton remains on a path to clinch the Democratic presidential nomination. But Trump got there first.
The New York businessman sealed the majority by claiming a small number of the party's unbound delegates who told the AP they would support him at the national convention in July. Among them was Oklahoma GOP chairwoman Pam Pollard.
"I think he has touched a part of our electorate that doesn't like where our country is," Pollard said. "I have no problem supporting Mr Trump."
It takes 1237 delegates to win the Republican nomination. Trump has reached 1239 and will easily pad his total in primary elections on June 7.
Many on the right have been slow to warm to Trump, wary of his conservative bona fides. Others worry about his crass personality and the lewd comments he's made about women.
Trump said during a press conference on Thursday that he would "absolutely" end his habit of attacking fellow Republicans now that the nomination is effectively his. But that truce appeared to be short-lived.
Speaking later at a rally in Billings, Montana, Trump said 2012 GOP nominee Mitt Romney, who has refused to endorse him, had "failed so badly".
His campaign also released a celebratory Instagram video that features a montage of former rivals, including Jeb Bush and Ted Cruz, saying he would never be the party's nominee.
Still, millions of grassroots activists, many of them outsiders to the political process, have embraced Trump as a plain-speaking populist.
He told a Bismarck audience that Clinton has "declared war on the American worker", that she's "going to abolish your right to own guns," and that she created a foreign policy legacy "of total chaos".
He said, "The choice in November is a choice between a Clinton agenda that puts donors first or an agenda that puts America first: my agenda."
