Donald Trump and Joe Biden are set for a high-stakes debate on Thursday that may be the final opportunity for the president, who is trailing in the polls, to present his case for re-election to a prime-time US television audience of millions.
Opinion polls show there are relatively few voters who have yet to make up their minds.
A record 42 million Americans have already cast ballots ahead of the debate in Nashville, Tennessee, meaning Mr Trump's window to influence the outcome of the race is narrowing.
The candidates intend to focus on undecided voters.
Mr Trump, 74, is expected to use the second and last debate to renew his attacks on the past business dealings of Mr Biden's son Hunter in China and the Ukraine.
Mr Trump and his aides have verified no evidence of the corruption allegations, which in the first debate his rival called false.
Mr Biden, 77, is expected to focus on the president's handling of the coronavirus crisis, which has left more than 222,600 people dead in the US and millions unemployed.
With the election just 12 days away, the debate may be Mr Trump's last and best chance to make up ground before the November 3 vote.
"This is probably one of the largest audiences they'll reach right before the election," said Amy Dacey, executive director of the Sine Institute of Policy and Politics at American University and former chief executive of the Democratic National Committee.
"I think the Trump team must be looking at this as a must to reach people and convince people."
Kyle Kondik, managing director of the political newsletter Sabato's Crystal Ball at the University of Virginia Centre for Politics, said: "The stakes are high for both candidates, though.
"Biden wants to keep the race just where it is and he doesn't want to provide any late fuel to the Trump campaign."
Mr Trump trails Mr Biden by 7.7 points in a RealClearPolitics average of national polls and is behind in most of the key battleground states that are crucial to victory, including Michigan and Wisconsin.
The latest Reuters/Ipsos poll showed the challenger ahead by nine points nationally, down from 12 points during the first week of October.
The 90-minute debate, due to start at 9pm Eastern Time, is being held at Belmont University in Nashville, in the southern state of Tennessee.
It will be televised by all of the major broadcast networks and cable news channels, and streamed live on platforms including YouTube.
The first debate on September 29 was a chaotic affair with constant interruptions and name-calling. and measures have been put in place this time to try to ensure order.
Each candidate's microphone will be cut off while the other answers questions from the moderator.
Swing voters, particularly women, were deterred by Mr Trump’s performance in the first meeting, post-debate polls and focus groups showed. More than 73 million people watched the debate.
A second debate planned for October 15 was cancelled after the president came down with Covid-19 and declined to take part in a virtual debate.
As a health precaution, Plexiglas barriers have been erected alongside the lecterns where the two candidates will stand.
The moderator, Kristen Welker, a White House correspondent for NBC News, picked the topics for the debate.
It will cover fighting Covid-19, American families, race in America, climate change, national security and leadership.
Mr Trump has attacked Welker on several occasions, calling her a "radical Democrat" and "no good".
He took another shot at her on Thursday as the White House released raw footage of an interview he did with the CBS show 60 Minutes.
"Look at the bias, hatred and rudeness on behalf of 60 Minutes and CBS," Trump said. "Tonight's anchor, Kristen Welker, is far worse."
In the interview, Mr Trump also continued to fling accusations of corruption at the Bidens.
"It's the biggest scandal," Mr Trump said. "I think it's one of the biggest scandals I've ever seen and you won't cover it."
The debate comes a day after former president Barack Obama hit the campaign trail for Mr Biden, delivering scathing criticism of Mr Trump and urging Democrats to not be lulled into complacency by his former vice president's lead in the polls.
Mr Obama issued a stark reminder of 2016, when surveys showed Hillary Clinton as the clear favourite, only for her to lose to Mr Trump on election day.
