American foreign policy for over half a century was defined by its bipartisan nature, symbolized in the old adage that politics stopped at the water's edge.

But in recent years, foreign policy has become as politically charged as virtually everything else in American life, and when voters mail in their ballots or go to the polls, they'll face a stark difference between the Republican and Democratic nominee and their views of the world and the United States' place in it.

Ahead of Thursday's debate, the Trump campaign made an eleventh-hour demand that the questions focus on foreign policy, as opposed to the topics chosen by the moderator, though the campaigns agreed months ago to allowing the moderator to choose the debate topics.

Read about how Trump and Biden differ on major foreign policy issues including relations with China, Iran, North Korea and NATO allies, among others, here.

-ABC News’ Conor Finnegan

Aides to Trump and Biden separately reported their candidates tested negative for COVID-19 ahead of the final debate.

Earlier Thursday, White House press secretary Kayleigh McEnany told reporters that Trump had tested negative, and Trump’s chief of staff Mark Meadows said the president was tested during the flight from the Washington, D.C., area to Nashville.

They did not offer information on whether the people who accompanied Trump to Nashville -- including Ivanka Trump, Jared Kushner, Tiffany Trump, Bill Stepien, David Bossie and Robert O’Brien -- were tested.

At a NBC News town hall last week, Trump claimed he didn’t remember whether he’d been tested on the day of the first debate, which took place before his first positive COVID-19 test was reported.

-ABC News' Molly Nagle and Jordyn Phelps

Atlanta Mayor Keisha Lance Bottoms tweeted Thursday that she will be attending the presidential debate as a guest of Biden. Bottoms also tweeted that she was a guest at the first debate in September.

Other guests of the former vice president include small business owners Zweli and Leonardo Williams of Durham, North Carolina. Biden campaign senior adviser Symone Sanders said the campaign invited them to draw attention to the struggles of small business owners.

“These small business owners from Durham -- because these are much like small business owners from Wisconsin, like the small business owners down in Georgia, like small business owners all over this country -- who are grappling with how to make ends meet, how to continue to provide, not only for their families but their employees. They're making decisions that, frankly, they shouldn't have to make because the president failed them,” Sanders said.

-ABC News’ John Verhovek and Beatrice Peterson.

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

In the wake of that canceled second showdown and a chaotic first debate before it, the Commission on Presidential Debates announced earlier this week it would mute candidates’ microphones at certain points Thursday to avoid interruptions and “maintain order.”

Trump and Biden will now have two minutes each of uninterrupted time to speak at the beginning of each 15-minute segment, of which there are six, in the 90-minute debate.

After the first four minutes of each segment, both of their microphones will go live for an “open-discussion portion,” the commission co-chair said. The moderator will not have control of the candidates' mics at any point. Those will instead be controlled by event production staff.

ABC News Chief Congressional Correspondent Mary Bruce said on ABC News Live Prime ahead of the debate, “Just because your microphone is down, doesn't mean that you necessarily will stop talking.”

Trump has attacked the integrity of the debate commission and its chosen moderators in recent weeks, deeming it all "crazy" and the new mic rule "very unfair” -- but sources have told ABC News Chief White House Correspondent Jonathan Karl that some advisers think muting the mics will actually help the president.

Biden, meanwhile, has called the muting of mics "a good idea" and said he’s expecting Trump to fire off personal attacks during their final showdown.

ABC News Chief Anchor George Stephanopoulos said the big question for Biden, who leads in nationwide polls, is whether he can close the deal with voters from Nashville. For Trump, it's how will he adjusts his performance after the "disastrous" first debate more than three weeks ago.

"Even the president's own aides called his blustering performance a self-inflicted wound," Stephanopoulos said.