Republican presidential candidate Donald Trump plunged into his final-week sprint to Election Day Monday unleashing a harsh new attack against Democrat Hillary Clinton in Michigan, a state that hasn't favored a Republican for president in nearly three decades.

His message was welcomed by supporters, but his location frustrated anxious Republicans who fear their nominee is riding his unorthodox political playbook too long, even as Clinton's developing email problems offer new political opportunity.

"Her election would mire our government and our country in a constitutional crisis that we cannot afford," Trump declared in Grand Rapids, pointing to the FBI's renewed examination of Clinton's email practices as evidence the former secretary of state might face a criminal trial as president.

Hillary Clinton forcefully challenged the FBI's new email inquiry Monday, declaring during a campaign rally in battleground Ohio, "There's no case here."

Clinton's comments were her most pointed yet on the subject, and they underscored her campaign's decision to fight back aggressively against FBI Director James Comey.

On Friday - just over a week from Election Day - Comey alerted Congress that the FBI has obtained new material that may be related to its dormant investigation into whether classified information passed through Clinton's private email server while she served as secretary of state.

The FBI plans to review the emails to see if they contain classified information and if so, whether they were handled properly. The Justice Department said Monday it would "dedicate all necessary resources" to concluding the review promptly.

Clinton accused the FBI of having jumped into the election "with no evidence of any wrongdoing with just days to go". She said that if the bureau wants to look at the emails, which appear tied to her longtime aide Huma Abedin, "by all means, they should look at them."

But she insisted the FBI would reach the same conclusion it did earlier this year, when it declined to recommend Clinton and her advisers face charges for how they handled classified information.

"They said it wasn't even a close call," she said. "I think most people have decided a long time ago what they think about all of this."

National polls show a tightening race. But with more than 23 million ballots already cast through early voting, it's unclear whether Trump has the time or capacity to dramatically improve his standing over the next week in states like Michigan, where few political professionals in either party expect a Republican victory on Nov 8.

The breakdown of those voters by party affiliation, race and other factors point to an advantage for Clinton.

Overall, more than 23 million votes have been cast, far higher than the rate in 2012, according to Associated Press data. That represents nearly 20 percent of the total votes expected nationwide, if turnout is similar to 2012. In all, more than 46 million people - up to 40 percent of the electorate - are expected to vote before Election Day.

Meanwhile, some Republicans are skeptical that the FBI's renewed interest in Clinton's email will erase the Democrat's advantage.

"It would take something like an indictment to turn it into a dead heat," Republican pollster Whit Ayres said.