Mrs Clinton – who has seen her national opinion poll lead shrink in recent days – targeted her Republican rival’s temperament and attitude to women.
Mr Trump claimed she would be followed into the White House by criminal investigations.
He was boosted by a rare campaign appearance by his wife Melania.
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In her first speech since July’s Republican convention, the former model spoke about being an immigrant and a mother and said her husband would “make America fair”.
She also vowed, in a speech in the Philadelphia suburbs, to lead a campaign against cyber-bullying if she becomes first lady, and to combat a culture that has “gotten too mean and too rough”.
She made no reference to her husband’s record of name-calling on social media.
Mr Trump has gained ground on Mrs Clinton in a number of swing states, such as Florida and North Carolina, according to polls. Some national tracker polls are now suggesting the two candidates are neck-and-neck.
The Reuters/Ipsos States of the Nation project estimates Mrs Clinton’s odds of winning the necessary 270 Electoral College votes on Tuesday at about 90%, down from 95% last week.
The momentum appears to be with Mr Trump, who has joked about having to stay on message, as the final weekend of campaigning approaches.
He has capitalised on a new FBI investigation into a Clinton aide’s emails.
“Here we go again with the Clintons – you remember the impeachment and the problems,” Mr Trump said at a rally in Jacksonville, Florida, his fourth appearance in the state.
“That’s not what we need in our country, folks. We need someone who is ready to go to work.”
Later, at a night-time rally in North Carolina, he delivered a speech on defence in which he said he could not imagine Mrs Clinton as commander-in-chief.
Mrs Clinton continued to focus on Mr Trump’s character, telling a rally in North Carolina: “He has spent this entire campaign offering a dog whistle to his most hateful supporters.
“This has never happened to a nominee of a major party.”
She added: “If Donald Trump were to win this election we would have a commander-in-chief who is completely out of his depth and whose ideas are incredibly dangerous.”
President Barack Obama has been working almost as hard as Mrs Clinton, as he attempts to rally support among young voters and African Americans.
Analysis of early voting returns suggests black voters may not be turning out in the kinds of numbers they did for Obama in 2012.
Alluding to Mr Trump’s past as a reality TV show host, Mr Obama told students at Florida International University in Miami: “This isn’t a joke. This isn’t Survivor. This isn’t The Bachelorette. This counts.”
Mrs Clinton also got a boost from her one-time rival for the Democratic nomination, Bernie Sanders. He appeared alongside her at a rally in North Carolina on Thursday evening, praising her commitment to increasing the minimum wage and tackling inequality.
Music star Pharrell Williams was also at the event, hailing Mrs Clinton’s record on women’s rights.