Donald Trump has said rival Hillary Clinton is disqualified to run for president and sharply criticised her record on trade, foreign policy, human rights and immigration.
The presumptive Republican nominee, who has denounced Mrs Clinton's $42m war chest as "blood money," sharpened his attacks on her after a series disastrous headlines fuelled speculation that his controversial campaign is unravelling.
"Hillary Clinton may be the most corrupt person ever to seek the presidency," he said in a speech at Trump SoHo, his five-star hotel in Manhattan to shouts of "Trump, Trump, Trump" from supporters.
He said the stakes in the November general election could not be higher and presented himself as an innovative thinker who could get things done and protect voters from an economic and political system rigged against them by career politicians, Mrs Clinton included.
Mr Trump attacked Mrs Clinton as a "world-class liar" who had "perfected the politics of personal profit and theft" and lacked the judgement to be the US's first woman commander-in-chief.
"She ran the State Department like her own personal hedge fund - doing favours for repressive regimes, and many others, in exchange for cash," he alleged of her tenure as Secretary of State.
He also alleged that her "disgraceful" foreign policy had cost the US "thousands of lives and trillions and trillions of dollars" in reference to her support for the invasions of Afghanistan and Iraq.
"In just four years, secretary Clinton managed to almost single-handedly destabilise the entire Middle East," he added, calling the self-styled Islamic State extremist group a threat because of her decisions.
On economics, he attacked her support for trade deals, saying they had wrought "total devastation" for working Americans and cost the country nearly a third of its manufacturing jobs.
The US's trade deficit with China had soared by 40% while she was secretary of state, he said, adding, "Hillary Clinton gave China millions of jobs and in exchange Hillary Clinton got rich."
Mr Trump claimed that Mrs Clinton and her husband, former president Bill Clinton, had made $153m in speeches to lobbyists, foreign governments and CEOs since 2001 and castigated her for refusing to release the transcripts of her closed-door speeches to Wall Street.
He also claimed that her family's Clinton Foundation had taken millions from countries in the Middle East that abuse women and members of the LGBT community.
British should 'go it alone'
Earlier, Mr Trump admitted he has not studied closely whether Britain should exit the EU - but he weighed in anyway, saying the British should "go it alone."
"I don't think anybody should listen to me because I haven't really focused on it very much," he told Fox News on the eve of the referendum on EU membership.
But he pointed to the "mess" of Europe's migration crisis, an issue that Mr Trump routinely brings up along with US immigration policy at his campaign rallies.
"When you look at the things that are going on over there my inclination would be go it alone and go back to where you came from," he told the network. "That's just my feeling."
Mr Trump has run a populist campaign attracting voters who are fed up with Washington and the political status quo. Many also support tighter immigration rules.
He has scheduled a trip to Scotland to attend the ceremonial re-opening of his golf course at Turnberry on Friday, the same day the Brexit results are expected.
"I'm right in the middle of it. I'll be over there just as the decision's coming in," the real estate mogul said.
Despite voicing his support for an exit, Mr Trump was adamant that the British make up their own minds.
"I want people to do what they want to do," he said.
He also insisted that the result would not help him either way. "Getting out or staying in makes no difference to me. It doesn't have any impact on me," he said.
"I also tell people don't go with the recommendation because it's a recommendation that I make - but that's where I stand."
Mr Trump is one of very few prominent international figures who have publicly expressed support for the UK leaving the EU.
Rubio seeks re-election to Senate, calls Trump 'worrisome'
Meanwhile, former Republican presidential candidate Marco Rubio said he would seek re-election to his US Senate seat, reversing a pledge not to run and saying the prospect of a Trump presidency worries him.
The first-term Florida senator said the Senate can be frustrating but said it is also a place where policy advances can be made and one that can act to counteract the power of the White House.
"But as we begin the next chapter in the history of our nation, there's another role for the Senate that could end up being its most important in the years to come: The Constitutional power to act as a check and balance on the excesses of a president," Mr Rubio said in a statement.
Republicans are more likely to retain Mr Rubio's seat if he is a candidate in the 8 November general election, as they seek to hold onto their majority in the Senate.
Senate Majority Leader Mitch McConnell said Mr Rubio running again was a "terrific outcome."
Mr Rubio had harsh words for Mrs Clinton warning that her election would mean four more years of what he termed President Barack Obama's "failed" policies.
But he also said the prospect of Mr Trump winning the White House was also "worrisome", citing the candidate's statements about women and minorities.
"If he is elected, we will need to be willing to encourage him in the right direction, and if necessary, stand up to him," Mr Rubio said.
The decision reverses a campaign pledge Mr Rubio made a year ago to retire from the Senate if his presidential bid were unsuccessful.
He ended his bid for the Republican presidential nomination in March after he lost the primary in his home state to Mr Trump.
Republican Representative David Jolly, who had been seeking the Senate seat, ended his campaign on Friday, opening the door for Mr Rubio.
The Democratic challenger, US Representative Patrick Murphy, criticised Mr Rubio for changing his plans.
"Marco Rubio abandoned his constituents, and now he's treating them like a consolation prize," Mr Murphy said in a statement.
Mr Rubio announced his re-election bid two days before a Friday deadline to tell state officials that he planned to run.
