The bitter and increasingly tight contest between President Donald Trump and Democrat Joe Biden turned on a handful of battlegrounds such as Pennsylvania and Michigan, dashing hopes for a decisive victory for either candidate.

Democratic presidential candidate Joe Biden's campaign said on Wednesday it has legal teams standing by if US President Donald Trump follows through with his threat to go to court to try to stop the counting of votes.

"If the president makes good on his threat to go to court to try to prevent the proper tabulation of votes, we have legal teams standing by ready to deploy to resist that effort," Biden campaign manager Jen O’Malley Dillon said in a statement.

Meanwhile, Arizona was the first state to flip in the presidential race; all other states with declared winners mirrored their 2016 outcomes. But several key swing states remained up in the air Wednesday morning.

The first batch of Arizona results posted Tuesday night, reflecting early votes, broke mostly for Democratic nominee Joe Biden, which was expected as Democrats turned out at higher rates before Election Day. But results from Tuesday’s voting were expected to favour President Donald Trump.

Biden also picked up an Electoral College vote in a Nebraska congressional district that voted for Trump four years ago. The president hadn't converted any new territory by early Wednesday morning.

Democrats' hope that Biden could bring the race to an early close by capturing Florida evaporated early Wednesday. Republicans breathed another sigh of relief when must-win Ohio was added to Trump's column. Biden made an unplanned visit to the Buckeye State on Monday, hoping the state might be within his reach on Election Day.