A combo image shows Donald Trump and Joe Biden. File/ AP

President Donald Trump squares off against Democratic challenger Joe Biden on Thursday in what promises to be a bruising final debate with less than two weeks left in their turbulent fight for the White House.

The pair's last televised showdown spiraled into an ugly confrontation marred by interruptions and name calling, prompting organizers to introduce mic-muting this time to try to keep things civil.

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The debate in the southern city of Nashville fires the starting gun on the final sprint to the November 3 election in a deeply polarized and tense United States fearful the results will set off court battles and more protests.

US President Donald Trump dances at the end of a Make America Great Again rally in Gastonia on Wednesday. AFP

Trump has stepped up his attacks on Biden, who national polls show leading the race, as he fights to hold on to the White House after four tumultuous years.

To cut down on the interruptions that marked the last debate, during the two minutes the candidates have to respond to the moderator's questions only their mic will be live.

"I think the mute is very unfair and I think it's very bad," Trump said this week, branding debate moderator Kristen Welker a "radical Democrat."

US Democratic presidential candidate Joe Biden speaks during a voter mobilisation event in Novi, Michigan. File/Reuters

Biden, 77, had no public events on his schedule on Wednesday for the third day in a row, as the 74-year-old Trump has been hitting multiple rallies per day.

Barack Obama was stumping for his former vice president on Wednesday, urging Democrats to not be lulled into complacency by Biden's lead in the polls.

He issued a stark reminder of 2016, when surveys showed Hillary Clinton as the clear favourite — only for her and her supporters to be shocked by a Trump victory on election day.