WASHINGTON (AP) – The fate of the United States presidency hung in the balance Wednesday morning, as President Donald Trump and Democratic challenger Joe Biden battled for three familiar battleground states—Wisconsin, Michigan and Pennsylvania—that could prove crucial in determining who wins the White House.

It was unclear when or how quickly a winner could be determined. A late burst of votes in Michigan and Wisconsin gave Mr. Biden a small lead in those states, but it was still too early to call the race. Hundreds of thousands of votes were also outstanding in Pennsylvania.

Both candidates spent months pressing dramatically different visions for the nation’s future and voters responded in huge numbers, with more than 100 million people casting votes ahead of Election Day.

But the margins were exceedingly tight, with the candidates trading wins in battleground states across the country. Mr. Trump picked up Florida, the largest of the swing states, while Mr. Biden flipped Arizona, a state that has reliably voted Republican in recent elections. Neither cleared the 270 Electoral College votes needed to carry the White House.

Vote tabulations routinely continue beyond Election Day, and states largely set the rules for when the count has to end. In presidential elections, a key point is the date in December when presidential electors met. That is set by federal law.

Several states allow mailed-in votes to be accepted after Election Day, as long as they were postmarked by Tuesday. That includes Pennsylvania, where ballots postmarked by November 3 can be accepted if they arrive up to three days after the election.

Pennsylvania Governor Tom Wolf tweeted that his state had over 1 million ballots to be counted and that he “promised Pennsylvanians that we would count every vote and that’s what we’re going to do.”

Democrats typically outperform Republicans in mail voting, while the GOP looks to make up ground in Election Day turnout. That means the early margins between the candidates could be influenced by which type of votes—early or Election Day—were being reported by the states.