Trump's advisers have urged him to highlight his policy differences with Biden and present his case to the American people as to why he deserves another four years in office, sources said.

He trails Biden in the polls and this is his final opportunity to pitch himself to a large audience in primetime before Election Day.

Some top advisers would like to see a less combative, calmer Trump at the debate but concede that is an uphill battle given the president will focus on Biden’s family, which aides hope will get under Biden's skin, multiple sources said. However, other top advisers don't think that focus helps move any of the remaining swing voters. 

Aides have also urged him not to interrupt Biden as much and have been re-watching the last debate with Hillary Clinton in 2016 as a guide as the White House views that debate as his strongest. The president's team also intends to heavily monitor the mute button usage, sources said. 

Trump has held no mock debates leading up to his final face-off with Biden and has prepped -- in what one source described as a "very compartmentalized" way -- with different subject matter experts providing the president a briefing and preparing notes for the president on a particular topic. He has said his best preparation comes from his exchanges with the press.

The group advising the president has also slimmed down. For example, his former counselor Kellyanne Conway and former New Jersey Gov. Chris Christie have taken less prominent roles.

-ABC News’ Katherine Faulders, John Santucci and Will Steakin

As he boarded his plane to Nashville earlier, Biden spoke briefly with reporters, telling them he's looking forward to the debate and said he hopes Trump plays by the rules.

"There's plenty of time to talk when this is over, OK? So hopefully he's going to play by the rules. Hopefully everyone's been tested. Hopefully it's all worked out, the way the rules are. I'm looking forward to this," Biden said before getting on the plane.

Biden's team has remained tight-lipped about their debate preparation for the final debate, but allies and advisers to Biden said they aren't expecting much of a change from either candidate ahead of the crucial matchup.

"I think Joe Biden is prepared for a completely unconventional debate in which the President of the United States does not act presidential (for) one minute. And the challenge is to not be distracted by the Trump show, and to make sure that Joe effectively puts out his positive vision," Sen. Chris Coons, D-Del., told ABC News.

An aide to Biden said the former vice president plans to again focus on his message to viewers at home, but would not be shy about standing up to Trump's interjections when necessary, especially given reports of the president's planned personal attacks on Biden and his family.

-ABC News' Molly Nagle

The debate between Trump and Biden will run from 9 p.m. to 10:30 p.m. with no breaks for commercials.

The program will be divided into six segments of 15 minutes each with topics decided by the moderator, NBC News White House Correspondent Kristen Welker.

Topics for Thursday’s debate are:

The debate commission has removed the two plexiglass barriers that were positioned between the candidates' lecterns.

Peter Eyre, senior advisor with the Commission on Presidential Debates, said that circumstances on the ground have changed and it's unlikely that the plexiglass partitions will be put back up, according to a pool report.

The news follows both candidates reporting testing negative for COVID-19. It's unclear if those who accompanied them to travel to Nashville were also tested.

-ABC News' Drew Millhorn

Trump said his administration is "trying very hard" to locate the 545 children whose parents can't be located after being separated from their parents. However, the president also said that some of the children were brought to the U.S. by "coyotes" and "cartels."

He then pivoted to say that the Obama administration built the detention centers where many migrants are held.

"They had a picture in a certain newspaper, there was a picture of these horrible cages. They said look at these cages. President Trump built them," Trump said. "Then it was determined they were built in 2014. That was him. They built cages."

In a fiery response, Biden emphasized that the children came to the U.S. with their parents.

"Coyotes didn't bring them over, Biden said. "Their parents were with them. They got separated from their parents, and it makes us a laughingstock and violates every notion of who we are as a nation.

Welker asked about the Obama administration's failure to deliver on immigration reform. Biden said that it took too long to get the policy correct.

"It took too long to get it right. Took too long to get it right," he said. "I will be president of the United States, not vice president of the United States. And the fact is, I've made it very clear, within 100 days, I going to send to the United States Congress a pathway to citizenship for over 11 million undocumented people and all of those so-called dreamers, those DACA kids, they are going to be immediately certified again to be able to stay in this country and put on a path to citizenship."

Trump responded that Biden "had eight years to do what he said he was going to do."

Biden and Trump then went back and forth over the catch-and-release policy.