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To win the presidency, candidates have to hit the magic number of 270 electoral votes. The Electoral College comprises a total of 538 members, with each state getting a total number of electoral votes equal to its congressional delegation and three additional electoral votes for District of Columbia.

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 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.