US President Donald Trump and Democratic rival Joe Biden made a last-ditch push for votes in battleground states on Monday.
It comes as their campaigns prepared for post-election disputes that could prolong a divisive presidential election.
Trump, who is trailing in national opinion polls, has continued to lob unfounded attacks on mail-in ballots, suggesting he would deploy lawyers if states are still counting votes after Election Day on Tuesday.
Trump told reporters on Monday evening that Pennsylvania's plans to count mail ballots that arrive up to three days after Election Day would lead to widespread cheating, although he did not explain how.
He urged the US Supreme Court to reconsider its decision that left the extension in place. The court has left that possibility open.
"Bad things will happen and bad things lead to other type things," he told reporters in Wisconsin, another battleground state.
On Twitter and Facebook, Trump said the court decision would "induce violence in the streets." The social media platforms flagged the posts, adding disclaimers saying the content "might be misleading" and that voting by mail and voting in person have a "history of trustworthiness".
Twitter and Facebook both said they would attach warning labels to messages that prematurely claim an election victory.
It is not unusual in the United States for states to take several days or even weeks to count their votes, and a record surge in mail ballots as a result of the coronavirus pandemic could draw out the process further this year.
"Under no scenario will Donald Trump be declared a victor on election night," Biden campaign manager Jennifer O'Malley Dillon told reporters.
Biden himself predicted a quick victory, but also sought to downplay the drama.
"I'm hoping for a straightforward, peaceful election, a lot of people showing up," he told reporters in Pittsburgh.
The election has prompted an unprecedented wave of litigation over whether to adjust voting rules in light of the COVID-19 pandemic.
Both sides have amassed armies of lawyers who are prepared to take on post-election battles.
On Monday, a federal judge in Texas rejected a Republican bid to throw out about 127,000 votes already cast at drive-through voting sites in the Democratic-leaning Houston area. But the legal uncertainty prompted officials to close most drive-through sites on Election Day.
Trump, 74, is seeking to avoid becoming the first incumbent president to lose re-election since fellow Republican George H.W. Bush in 1992. Despite Biden's national polling lead, the race in swing states is seen as close enough that Trump could still piece together the 270 votes needed to prevail in the state-by-state Electoral College system that determines the winner.
"I think tomorrow is going to be one of the greatest victories in the history of politics," he told a large crowd in Grand Rapids, Michigan - the same place he concluded his 2016 presidential run.
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