MICHIGAN/KENT: Republican presidential candidate Donald Trump called Democratic rival Hillary Clinton a threat to the country on Monday, saying that if she is elected a probe into her emails could shadow her entire term in office, as the latest Reuters/Ipsos poll showed Clinton's lead narrowing slightly.

"The investigation will last for years. The trial will probably start," Trump told a rally in Grand Rapids, Michigan. "Nothing will get done. I can tell you, your jobs will continue to leave Michigan. Nothing’s going to get done.”

The Federal Bureau of Investigation on Friday said it was investigating newly discovered emails that might relate to Clinton's use of a private email server when she was secretary of state.

Clinton on Monday again said she was confident that the FBI would not find anything problematic in her emails and would reach the same conclusion they did earlier this year.

"It wasn't even a close call," she said at a rally in Cincinnati, Ohio, of the FBI investigation.

But just eight days ahead of the election - a time when candidates typically feel that the hard work of the campaign is behind them - both Clinton and Trump have ratcheted up their attacks on the other's character and fitness for office.

Clinton, who had been riding high in opinion polls in recent weeks as Trump was hit by fallout from the release of a 2005 video in which he bragged in vulgar terms of groping women, now finds herself on the defensive.

Trump is hoping to convince voters that electing Clinton would prompt "a constitutional crisis that we cannot afford," as her emails would be subject to years of controversy, in the wake of the FBI's announcement on Friday that it continues to investigate material possibly related to her emails.

Clinton on Monday continued to level attacks against Trump's ability to control nuclear weapons.

"I am running against someone who says he doesn't understand why we can't use nuclear weapons," she said in Cincinnati. "He wants more countries to have nuclear weapons.

"I wonder if he even knows that a single nuclear warhead can kill millions of people," she added.

Little is publicly known yet about the emails being investigated, other than that they were found during an unrelated probe into the estranged husband of a top Clinton aide.

FBI Director James Comey told members of Congress on Friday the agency was probing more emails that might relate to Clinton's use of a private email server, but added, "We don't know the significance of this newly discovered collection of emails."

Trump, who has repeatedly referred to Clinton as "corrupt Hillary," on Monday said the email probe shows what a poor role model she is - seemingly trying to turn the tables on Clinton, who has assailed his character over disclosures of vulgar comments he made about groping women.

"I want to tell you, she is a terrible example for my son and the children of this country,” he said in Warren, Michigan, mentioning his youngest son, Barron. “Hillary is the one who broke the law over and over and over again.”

CLINTON HOLDS NARROW LEAD

Until the Friday revelation, Clinton had been coasting with a comfortable lead over Trump.

Opinion polls now shows Clinton's lead over Trump has narrowed slightly since early last week. It is not yet known if the email controversy will hurt her support. Millions of Americans have already cast their ballot in early voting.

Clinton holds a 5 point lead over Trump in the latest Reuters/Ipsos poll, receiving 44 percent of likely voters compared to Trump at 39 percent.

Despite the controversy about her email, Clinton continues to hold a large advantage in the Electoral College, the process that selects a president by awarding votes through individual state elections. Clinton holds leads in several key swing states, including Pennsylvania and New Hampshire, where Trump must erode a large lead to be victorious.

'EXPLOSIVE'

The FBI spent a year investigating Clinton's use of a private email server, instead of government systems, while she was secretary of state from 2009 to 2013. Comey concluded in July that while Clinton and her staff had been "extremely careless" in handling classified information there were no grounds for any charges.

Comey, roundly criticized by Republicans for his decision not to recommend charges against Clinton at the end of the FBI probe in July, has now drawn the ire of senior Democrats. US Senator Harry Reid, the Democratic leader in the Senate, accused him of "a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another."

He said, without providing evidence, that the FBI was keeping “explosive information” under wraps about ties between the Trump campaign and Russian officials.

Clinton campaign manager Robby Mook called reports that the FBI would not discuss whether the Russian government was behind the hacking of Democratic email accounts because it was too close to the election "a blatant double standard."

In an August letter, Reid asked Comey to investigate whether Trump allies have worked with the Kremlin to influence the election, citing reports that a foreign-policy adviser had met with Putin allies on a July trip to Moscow and longtime Republican operative Roger Stone had been in touch with WikiLeaks.

The White House steered clear on Monday of direct criticism of Comey, who was appointed by Democratic President Barack Obama in 2013. Obama views the FBI head as a man of integrity and does not believe he is secretly trying to influence the outcome of the election, White House spokesman Josh Earnest told reporters.