Democrat challenger Joe Biden has a narrow lead in Wisconsin after officials completed their vote count in the pivotal state and the campaign of US President Donald Trump sought a recount.
Biden also led in another critical midwestern battleground state - Michigan - as he and Trump raced to get to the 270 electoral votes in the state-by-state electoral college needed to win the White House.
Trump won Wisconsin and Michigan in his 2016 election victory.
Wisconsin officials finished their tally at about midday local time after an all-night effort, showing Biden with a lead of just over 20,000 votes, or 0.6 per cent, according to Edison Research.
The Trump campaign immediately said it would seek a recount, which is permitted under state law when the margin is below 1.0 per cent.
A Biden victory in Wisconsin would significantly narrow Trump's path to a second four-year term, though the outcome remained in doubt with Michigan and other closely contested states including Pennsylvania, Arizona, Nevada, Georgia and North Carolina still counting votes.
Trump led in the southern states of Georgia and North Carolina as well as in Pennsylvania, where more than 1 million ballots were yet to be processed.
But if Trump loses Wisconsin, he would have to win all three as well as either Arizona or Nevada, where Biden was leading in the latest vote counts.
At the moment, not including Wisconsin, Biden leads Trump 227 to 213 in electoral college votes, which are largely based on a state's population.
In duelling conference calls with reporters earlier on Wednesday, officials from each campaign insisted their candidate would prevail.
"If we count all legal ballots, we win," Trump campaign manager Bill Stepien said, potentially setting the stage for post-election litigation over the counting of mail-in ballots.
Biden campaign manager Jennifer O'Malley Dillon told reporters the former vice president was on track to win the election while senior legal adviser Bob Bauer said there were no grounds for Trump to invalidate lawfully cast ballots.
"We're going to defend this vote, the vote by which Joe Biden has been elected to the presidency," said Bauer, adding that the campaign's legal team was prepared for any challenge.
Biden was expected to deliver an address later on Wednesday.
