“If you disrespect women before you’re in office, you will disrespect women while you’re in office. If you accept the support of Klan members,” Obama continued, “then you will tolerate that support when you’re in office.”

Mr Trump told Americans who cast their ballots for Mrs Clinton before the latest email scandal emerged that it may not be too late to change their vote.

Early voting was underway in 37 states before the FBI announcement, which threw her campaign into chaos and led to her poll numbers plummeting.

Most of the more than 15 million votes cast before the announcement are final, but seven US states actually allow voters to amend their ballots.

Mr Trump took to Twitter to spread the word:

"Now that you see that Hillary was a big mistake, change your vote to MAKE AMERICA GREAT AGAIN!" he wrote.

Wisconsin voters can cast up to three ballots, and multiple people in Winnebago County had already changed their votes according to Sue Ertmer, the county clerk.

"I don’t believe they even have to give a reason. They could go to the clerk and say that they basically need to cast a different ballot," she told WBAY.

A voter could change their mind again on Election Day, turning up at a polling station to nullify their second ballot, and receive a third.

Michigan and Pennsylvania, key battleground states, do not allow early voting but residents who have cast absentee ballots could change their choice in person on Election Day itself.

The Trump campaign announced on Wednesday it had raised $100 million in October from "small-dollar" donors.

His campaign announced that the total was garnered from 1.6 million small-dollar donations, a term traditionally used to describe contributions of less than $250.

Mr Trump has faced a significant fundraising deficit compared with his Democratic rival. In the first 19 days of October, Mr Trump raised about $30 million, compared with Mrs Clinton's $53 million.