Democrat Joe Biden captured the US presidency on Saturday, several major television networks said, as voters narrowly rebuffed Republican incumbent Donald Trump's tumultuous leadership and embraced Biden's promise to fight the coronavirus pandemic and fix the economy in a divided nation.

When the former vice president enters the White House on Jan 20, the oldest person to assume the office at age 78, he likely will face a difficult task governing in a deeply polarized Washington, underscored by a record nationwide voter turnout in a fight to the finish.

His projected victory came after four days of nail-biting suspense over the outcome of Tuesday's election, with the counting of votes in a handful of battleground states ongoing thanks to a flood of mail-in ballots.

Biden said on Friday he expected to win the race but stopped short of giving a victory speech. A Trump adviser acknowledged on Friday that the race had tilted against Trump, but said the president was not ready to admit defeat.

Biden had a 273 to 214 lead in the state-by-state Electoral College vote that determines the winner, having won Pennsylvania's 20 electoral votes to put him over the 270 he needed to secure the presidency, according to Edison Research.