Pennsylvania could finalise its election vote count as early as Thursday, a senior official in the pivotal battleground said, with Democrat Joe Biden winning the White House if the state's votes go his way.

Biden leads President Donald Trump in the all-important electoral vote count that determines the winner of the election, and prevailing in Pennsylvania would put the challenger over the magic threshold of 270 electoral votes needed to win -- regardless of the outcome in other states yet to be called.

"I think we definitely could" know a winner in Pennsylvania by Thursday, the state's Secretary of State Kathy Boockvar told CNN.

"Counties are furiously at work and it is looking like we are ahead of schedule," she added.

"I have been saying we'll have the overwhelming majority counted by tomorrow, but it's looking like we'll have the overwhelming majority counted by today."

When Boockvar spoke to CNN she said an estimated 550,000 ballots remained to be counted. Biden was trailing Trump by 108,772 votes out of 6,442,296 Pennsylvania ballots counted as of 2:45 pm (1945 GMT).

But the gap has been narrowing steadily as the state counts mail-in ballots, which have mostly favored Biden.

The former vice president has racked up at least 253 electoral votes out of 538 at stake. Pennsylvania's 20 votes would put him over the winning threshold of 270.

Demonstrators for both Biden and Trump have converged on Philadelphia, Pennsylvania's largest city, as the state's vote comes down to the wire.

If Trump holds Pennsylvania, Biden could still prevail if he wins at least two of the as-yet undecided battlegrounds of Arizona, Georgia, Nevada and North Carolina.

Trump has launched legal challenges in multiple states including Pennsylvania in an effort to halt or invalidate the count of some mail-in ballots.