As months of President Donald Trump undercutting the legitimacy of mail-in votes gave way to promises that he would challenge them in court, both sides made a final push to ensure their supporters turned out, even with the lingering threat of lawsuits aimed at invalidating ballots.

"Do not put ballots in the mail. Hand-deliver your mail ballot to your county election office, satellite election office or other designated drop box or drop-off location," Pennsylvania's top election official, Secretary of State Kathy Boockvar, a Democrat, said on Monday. "Do it today. Do not wait."

About 700,000 of some 31 lakh requested mail ballots in Pennsylvania remained outstanding, and some voters like Paige Waenke appeared to take Boockvar's message to heart.

Waenke, a 25-year-old who works in human resources, requested a mail ballot but worried about it getting lost in the mail or not being accepted. She dropped off her vote for Biden on Monday at a Montgomery County drop box in King of Prussia outside Philadelphia.

"I want to get our country back to a place where there is less fighting," she said.

Roughly 300 lawsuits already have been filed over the election in dozens of states across the country, many involving changes to normal procedures because of the coronavirus pandemic, which has killed more than 230,000 people in the US and sickened over 90 lakh.

Pennsylvania, where the candidates appeared to be divided by a razor-thin margin for the state's 20 electoral votes, seemed likely to be the epicentre of any post-election litigation. The deadline for receiving and counting absentee ballots is Friday under an extension ordered by the state's top court, a ruling that the Supreme Court left in place but suggested it would be open to revisiting.

Trump called the decision a "horrible thing" and warned "as soon as that election is over, we are going in with our lawyers". He added: "If people wanted to get their ballots in, they should have gotten their ballots in long before that."

Pennsylvania Attorney General Josh Shapiro, a Democrat, insisted in an interview on Monday that all votes would be counted and said he was encouraged that the state prevailed in lawsuits filed by Trump and his supporters: "We have lawyers and agents fanned off all across Pennsylvania prepared to deal with whatever may come." In a tweet, he expressed confidence as he taunted Trump: "If your lawyers want to try us, we'd be happy to defeat you in court one more time."

The Supreme Court's openness to revisiting its prior ruling, though, and the presence of a new Trump appointee on the high court, Justice Amy Coney Barrett, left others feeling anything but certain.

"Make sure, regardless what happens with litigation, that your vote is counted," said Sylvia Albert, director of voting and elections for Common Cause, which called for Barrett to recuse herself from any potential election litigation. "To me, it seems a clear voter-suppression tactic and an effort to invalidate ballots" that a party does not think are for them.

"I would hope the court would see that as an obvious power grab, but the reality is, we do not know what the court would do," Albert said.

Stung by 2016 and conscious that voter surveys could be wrong, the same note of caution pervaded the country a day ahead of an anything-but-typical election.

In Wisconsin, which Trump won by fewer than 23,000 votes four years ago, both sides called on supporters to get out to vote after a Supreme Court ruling last week rejected Democrats' attempt to allow ballots postmarked by Election Day to be counted for six more days.

All absentee ballots must be received in Wisconsin by the close of polls on Tuesday night. About 175,000 of more than 12 lakh requested ballots remained outstanding. At news conferences and public events and on social media campaigns, voters were told not to count on the mail.

"All our efforts right now are focussed on doors, phones, texts and emails," said Wisconsin Republican Party spokesman Alec Zimmerman, urging voters to do in-person balloting or use the 39 drop boxes around the state.

State Attorney General Josh Kaul, a Democrat, said the number of outstanding mail ballots was not a cause for concern, saying some people who request them "ultimately decide to vote in person".

In Michigan, 32-year-old Darnesha Coleman stood in a long line on Monday alongside her 80-year-old grandmother to vote at a Detroit polling place after the absentee ballots they requested never showed up. She worries that voting changes forced by the pandemic could jeopardise the election's outcome. "It is quite chaotic," she said. "It is not like it usually is. I do not think it is going to go like it is supposed to."

The Postal Service said it could not guarantee ballots mailed after October 27 would arrive by Election Day, a message at the heart of candidates' last-minute pleas for support.

"GEORGIA," tweeted Jon Ossoff, a Democratic candidate for the Senate. "Make your plan to vote ASAP. Find your absentee ballot Drop Box. If you requested an absentee ballot and still haven't gotten it, vote in-person on Election Day (Tuesday, Nov 3) instead."

Democrats have requested far more mail ballots than Republicans, but have also generally returned them at a brisker clip, making outstanding ballots in many states closely divided between the parties.

Whatever uncertainty remains about Tuesday, many people are resigned to ongoing disputes. Brookings Institution elections expert Elaine Kamarck sees litigation as inevitable, even as she says most voters should be "not very concerned" about getting their ballots counted because the law is on their side.

"The Constitution is pretty damn clear: States have the authority for running elections and particularly presidential elections. So as long as a state makes clear what its parameters are, the courts are loath to intervene," said Kamarck, a member of the Democratic National Committee. "I think the way voters should look at this is if it is a razor-thin race in some state, and that state makes the difference for one candidate or another getting to 270 electoral votes, then every single thing is going to be litigated."