This combined photo shows Vice President Kamala Harris (right) and former President Donald Trump. (AFP-Yonhap)

US Democratic presidential candidate Kamala Harris and her Republican rival Donald Trump are bracing for this week's high-stakes election showdown, with various polls showing them in a tight margin-of-error race.

Voters will go to polling stations nationwide on Tuesday to pick a new president. More than 75 million Americans had already cast an early ballot, data from the University of Florida showed. The number of eligible voters is estimated at about 244 million, according to the Bipartisan Policy Center.

Uncertainty still shrouds the general election as Vice President Harris and former President Donald Trump have been caught in a dead heat, with narrow margins in battleground state races that will likely determine the presidency.

To occupy the Oval Office, a candidate must win at least 270 electoral votes out of the total 538 electors representing the 50 states and the District of Columbia. Seven swing states -- Pennsylvania, Michigan, Wisconsin, Georgia, Arizona, Nevada and North Carolina -- are expected to play an outsize role in hitting that magic number.

Pennsylvania has been at the center of election-year attention as the Keystone State has 19 electoral votes -- the highest among the seven toss-up states -- that could provide a substantial boost for either of the candidate to surpass the 270 Electoral College threshold.

Harris is thought to secure 226 electoral votes from Democratic or Democratic-leaning states, while Trump is forecast to lock up 219 electoral votes from states traditionally supportive of Republicans.

Thus, Harris and Trump need to win 44 and 51 more votes, respectively, from swing states to hit the 270-mark.

Should Harris take Pennsylvania's 19 votes, she would need 25 more votes -- possibly from Michigan with 15 votes and Wisconsin with 10 votes -- to reach the magic number. For Trump, taking Pennsylvania will greatly brighten his election chances as well, though there are other scenarios for his possible win.

This is why both Harris and Trump have devoted a big chunk of their time and financial resources to courting undecided voters in Pennsylvania.

Trump lost Pennsylvania to Biden by a small margin in 2020, but won it by a thin margin in 2016 -- a reason why the state has been seen as a toss-up. Polls have shown Harris ahead in Michigan and Wisconsin.

Given the close race, questions have lingered over whether a smooth transition of power can happen as Trump has claimed he would only lose if Democrats "cheat" -- remarks that further reinforced speculation that he might not accept an election outcome in favor of his rival.

Some observers have worried that the US could undergo a period of post-election confusion, amid still potent memories of an attack on Capitol Hill on Jan. 6, 2021, though legal and other efforts have been made to forestall such an incident.

It remains uncertain when a winner will be named. In the 2020 election that proceeded amid COVID-19, President Joe Biden's victory was announced four days after the vote. But the 2016 election was called for Trump in the wee hours of the morning after Election Day.

Aside from the presidential vote, 34 Senate seats and all 435 House seats are at stake in Tuesday's general election. (Yonhap)