Hillary Clinton ramped up her pressure on Donald Trump in the election's most competitive states on Friday with an emotional TV ad targeting his criticism of a Muslim-American family.
Mr Trump, meanwhile, vowed to go all-out in the final three weeks so he'll have no regrets - even if he loses.
The nominees retrenched behind familiar arguments a day after appearing together at a charity event that veered into cutting personal attacks.
Mrs Clinton's new ad features Khizr Khan, whom Mr Trump assailed after Mr Khan spoke at the Democratic National Convention.
In the minute-long ad, which Mrs Clinton's campaign said was airing in seven battleground states, MrKhan retells how his son, Captain Humayun Khan, died in Iraq seeking to protect his US military unit from a suicide bomber.
"Mr Trump, would my son have a place in your America?" the father asks, welling up as the ad fades to black.
Mrs Clinton has sought to use Mr Trump's widely panned attacks on the father and his wife as evidence supporting her case that the Republican bullies minorities, especially those who disagree with him.
In person, she was promoting early voting in Ohio and planned a meeting with two activists for Black Lives Matter.
Mr Trump has settled on a new target: Michelle Obama, who has emerged as one of the most effective voices for Mrs Clinton.
One of the country's most popular Democrats, the first lady for years has been loathe to devote significant time to campaigning, but has done so in recent days with searing indictments of Mr Trump's treatment of women.
"All she wants to do is campaign," Mr Trump said as he rallied supporters in North Carolina. He cited comments Mrs Obama made during her husband's 2008 campaign in which she said someone who can't run their own house can't run the White House. "She's the one that started that," Trump said.
Mr Trump was unusually candid about the possibility of losing the election, a prospect that's grown in likelihood as Mr Clinton solidifies her lead in battleground states that will decide the election.
He said he is packing his schedule with campaign events through to Election Day so he will know he spared no effort even if ultimately unsuccessful.
"I will be happy with myself," he said.
The two candidates were still sharply at odds over Mr Trump's unprecedented assertion in the final debate on Wednesday that he may not concede if he loses.
Mrs Clinton has called that "horrifying," while Mr Trump has said he's merely reserving the right to contest the results if the outcome is unclear or questionable.
Underpinning his threat is his contention - presented with no evidence - that the election is "rigged" against him and may be soiled by widespread voter fraud. He's urged supporters to "monitor" polling places for potential shenanigans.
Fanning those flames, Russia's government has asked Oklahoma and two other states to allow Russian officials to be present at polling stations on Election Day, to study the "US experience in organisation of voting process".
Allegations by the US government that Russia is trying to influence the election by hacking Democratic groups has fed a Clinton camp claim that Russian President Vladimir Putin is siding with Mr Trump.
The Oklahoma secretary of state's office said it had denied the Russian request, in line with state law. At the White House, press secretary Josh Earnest said it was unclear what Moscow was trying to do.
"It's appropriate that people might be suspicious of their motives," Mr Earnest said.
Early voting is under way in more than 30 states.