(The Center Square) – It's Decision Day in the U.S. as millions of voters head to the polls to cast their ballot for president.

Will Americans decide that Republican President Donald Trump deserves another four years in office? Or will they turn to Democrat Joe Biden?

If it's a relatively close race, it's possible – maybe even likely – that we won't know a winner for days, or weeks even.

That's in part because more than 96 million Americans already voted early or by mail as of Monday. States have different rules for counting and processing mail-in votes. Some wait until after all polls close in the state. Some states also will accept mail-in ballots for days after Nov. 3.

In 17 states, mail-in votes were being counted before Election Day, according to Ballotpedia. In 16 states, mail-in votes can start being counted on Election Day before polls close. In the remaining 17 states, mail-in votes can't be counted until after polls close. Some states require election clerks to match the signatures on mail-in votes with signatures already on file, making the process more time-consuming.

In the key swing state of Pennsylvania, for example, some counties don't expect to start counting mail-in ballots until Wednesday or later. During Pennsylvania's June primary, roughly half of counties were still counting ballots a full week after Election Day.

Ohio, North Carolina, Georgia, Florida, Michigan, Wisconsin, Iowa, Arizona – even Texas – could also see close election results where a winner won't be declared immediately.

Trump or Biden need to take at least 270 electoral votes to secure the victory. In 2016, Trump won the electoral vote and the presidency despite losing the popular vote to Hillary Clinton by nearly 3 million. By winning the key swing states of Florida, Pennsylvania, Ohio, Michigan and Wisconsin, Trump claimed 304 electoral votes to Clinton's 227.

Polls opened as early as 5 a.m. eastern time in parts of Vermont, according to Ballotpedia, though most East Coast states' polls open between 6 and 7 a.m. The last U.S. polls close in Hawaii at 7 p.m. local time, which is midnight eastern.