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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.

The final topic of the night was climate change with Welker first asking each candidate how he would combat climate change and support job growth at the same time as president. Trump answered first.

"So we have the trillion trees program. We have so many different programs. I do love the environment, but what I want is the cleanest, crystal clear water, the cleanest air," Trump said, taking credit for the country's reduced carbon emission last year.

He went on to slam other counties air-quality and tout the U.S. by comparison.

"Look at China, how filthy it is. Look at Russia. Look at India. It's filthy. The air is filthy. The Paris Accord, I took us out because we were going to have to spend trillions of dollars, and we were treated very unfairly," Trump said, closing his two minutes by saying his administration has done an incredible job "environmentally."

When it was Biden's turn, he pivoted to the existential threat he says global warming poses to humanity and said the U.S. has a moral obligation to "deal with it" -- but said climate change also creates a prime opportunity for new jobs.

"Here's where we have a great opportunity. I was able to get both all the environmental organizations as well as labor, the people worried about jobs, to support my climate plan. Because what it does -- it will create millions of new good-paying jobs," Biden said. "Wall Street firms indicated that my plan -- my plan will, in fact, create 18.6 million jobs, 7 million more than his. This is from Wall Street. And I'll create $1 trillion more in economic growth than his proposal does. Not on climate, just on the economy."

Trump slammed Biden's response, claiming the former vice president's plan would cause a massive economic depression and deeming it a "pipe dream," before raising Biden's stance on fracking.

In the face of Trump's criticisms, Welker asked Biden directly, "Would you rule out banning fracking?"

"I do rule out banning fracking," Biden said. "We need other industries to transition to get to ultimately a complete zero emissions by 2025. What I will do with fracking over time is make sure we can capture the emissions from the fracking, capture the emissions from gas. We can do that and we can do that by investing money. It's a transition to that."

"Excuse me," Trump interrupted. "He was against fracking. He said it. I will show that to you tomorrow."

"Fracking on federal land, I said," Biden responded. "No fracking and/or oil on federal land."