Buoyed by a new poll showing him taking the lead, Mr Trump tells Democrats in Wisconsin that they can change their votes.
The ABC/Washington Post poll put Mr Trump on 46%, the first time he has led in the poll since May.
As the US presidential election enters the final days of campaigning, Mrs Clinton fought back, labelling her Republican rival a "bully" for his criticism of women's looks.
She was introduced on stage by former Miss Universe, Alicia Machado, who has claimed Mr Trump once called her "Miss Piggy" after she gained weight.
She said Mr Trump had been "cruel" and claimed she had spent years "fighting back eating disorders" as a result of his comments.
US President Barack Obama echoed that sentiment at a rally in Columbus, Ohio.
Mr Obama suggested that sexism is causing men to support Mr Trump over the woman attempting to become the first female president in US history.
"Has she made mistakes?" he asked the enthusiastic crowd. "Of course. So have I. There's nobody in the public arena over the course of 30 years who doesn't make some.
"She is a fundamentally good and decent person who knows what she's doing."
Meanwhile, Mrs Clinton focused her efforts on a tightening race in Florida, joined by her husband, Bill.
At a rally in Sanford, she concentrated on drawing sharp contrasts between herself and her Republican rival on issues including terrorism and national security.
"I know what happened not far from here at the Pulse nightclub in Orlando.
"I was in New York City on 9/11 as one of the two senators," she told the crowd at a rally in Sanford, Florida.
Mr Trump, meanwhile, appeared to be energised by the positive polling.
He told Democrats in Wisconsin that if they had already cast their ballots for Clinton, they should change them - and that state law allowed them to do so.
At a rally in Zorn Arena, he said that if Mrs Clinton were elected her legal troubles would continue.
"She is likely to be under investigation for many years, probably concluding in a very large-scale criminal trial," he claimed, before urging early voters to shift their allegiances, "with buyer's remorse".
Mr Trump appears to be trying to expand the electoral map of Wisconsin, where he's been trailing Mrs Clinton for months.
Rather than focusing on areas where the polls are closer, such as North Carolina, Florida, Nevada and Ohio, his team seem newly emboldened, banking on a low Democratic turnout.
:: Watch a special report from Ed Conway, Reversal Of Fortune, at 11.30 this morning, and at 2.30pm and 9.30pm.
:: There will also be continuing coverage from the campaign trail throughout the day on Sky News. Plus every night this week, we will have a special programme - America Decides - from midnight with Jeremy Thompson.