Donald Trump warned on Monday that the United States would be plunged into a "constitutional crisis" should Hillary Clinton win next week's presidential election.
"Hillary is likely to be under investigation for a very long time," the Republican told supporters in Grand Rapids, Michigan as he seized on revelations that the FBI was studying newly-discovered emails that may shed light on Clinton's use of private email as secretary of state.
"She's unfit and unqualified to be the president of the United States," Trump said.
"Her election would mire our government and our country in a constitutional crisis that we cannot afford."
Trump, 70, said he believed that if Clinton were elected, the American people would be subjected to the unprecedented scenario of a commander in chief facing criminal charges.
"We would have a criminal trial for a sitting president," Trump said.
The Republican's attacks have grown more strident since FBI director James Comey dropped a bombshell by announcing that the bureau will study thousands of newly discovered emails, reportedly after agents seized a laptop used by Clinton's close aide, Huma Abedin, and her now estranged husband Anthony Weiner.
With Comey under withering fire from Democrats for making the announcement so close to Election Day, Trump embraced him for having "guts" to move as he did.
"I really disagreed with him, I was not his fan," said Trump, who had fiercely criticized Comey for deciding back in July not to recommend prosecution against Clinton for her private email use.
"What he did (with his latest announcement), he brought back his reputation," Trump said.
The FBI's renewed focus reportedly stems from a probe of Weiner, a disgraced former congressman who is under investigation over allegations he sent sexual overtures to a 15-year-old girl.
"Thank you Huma. Good job Huma. Thank you Anthony Weiner," Trump sneered, to a roar and applause by the rowdy crowd.
Meanwhile the White House opted for caution in reacting to Comey, with spokesman Josh Earnest saying he would "neither defend nor criticize" the FBI director.
Earnest described him as a "man of integrity" but also pointed to norms that "limit public discussion of facts that are collected in the context" of ongoing investigations.