“Everything you care about, everything I care about and have worked for, is at stake,” Mrs. Clinton said, without mentioning Mr. Trump.

Mrs. Clinton has delivered a broad message of national unity in the campaign’s closing days, offering herself to voters as an avatar of tolerance and reconciliation, in contrast to Mr. Trump. She campaigned on Sunday in New Hampshire with Khizr Khan, the father of an Army captain slain in Iraq, whose speech in July castigating Mr. Trump as biased against Muslims and immigrants electrified the Democratic National Convention. Mr. Khan branded Mr. Trump as a figure of exclusion and division, asking of him, rhetorically: “Would anyone who is not like you have a place in your America?”

Mr. Trump, for his part, warned voters that they would never again see a candidate like him within reach of the presidency. At a rally in Sioux City, Iowa, he said repeatedly that he represented a “last chance” for voters angry about trade and immigration.

“The media, Wall Street and the politicians are trying to stop us because they know we will fix the rigged system,” he said.

The announcement from James B. Comey, the F.B.I. director, reaffirming his assessment that Mrs. Clinton should not be charged with a crime over her handling of classified information, came as a blow to Mr. Trump and other Republicans hoping that a pre-election bombshell would upend Mrs. Clinton’s campaign.