Milwaukee, Wisconsin - Kamala Harris headed to the swing state of Wisconsin on Thursday as she and rival Donald Trump vie for the remaining fraction of undecided voters in a deadlocked US presidential election.

The candidates are racing toward the Election Day finish line, with the Democratic vice president narrowly leading her Republican rival nationally and in several crucial swing states, although most polls are within the margin of error.

Both have been desperate to peel off support from each other in the final weeks of the race.

Harris planned to woo young voters and blue-collar workers in the manufacturing hub of La Crosse and in Green Bay, one of Wisconsin's largest cities.

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The vice president's momentum in the polls has plateaued in recent weeks, and both candidates have been on a blitz of new and traditional media as they try to win over the small number of undecided voters.

Harris sat down with right-wing Fox News on Wednesday in Pennsylvania, where she experienced her toughest cross-examination so far – taking several hits on her policy record and dodging some questions.

The vice president was pressed hard on when she noticed that Biden was mentally "diminished," how many immigrants had entered the country illegally, and whether she would apologize to the parents of a child murdered by undocumented migrants.

But Harris was able to pivot repeatedly to attacking "unstable" Trump, giving Fox News viewers a rare insight into his worst behavior and rhetoric – something that could sway disaffected Republicans and swing voters.

Her best moment came when she berated host Bret Baier for whitewashing Trump's recent threat to set the military on his political opponents, after Fox played a clip of the Republican cleaning up his remarks instead of the threat itself.

Republicans claimed the interview was a disaster while Democrats called it a triumph.