President Donald Trump is set to enter the first presidential debate on Tuesday down in the polls, but coming off the high of nominating his third Supreme Court justice. Some top advisers expect a dominant performance by the president, despite the opposing narratives of attacks on Joe Biden's mental state and the Trump campaign's work to raise expectations for the Democrat ahead of the showdown.

With days to go the debate, Trump has carried out conflicting and confounding strategies both repeatedly targeting Biden's mental acuity, claiming at a recent rally he doesn't know he's alive, while also making passing attempts to boost the former vice president as a seasoned debater who could outshine him.

"This guy doesn't have a clue. He doesn't know where the hell he is," Trump told his supporters in Pittsburgh a week to the day before the first debate. "This guy doesn't know he's alive."

At a campaign rally Saturday night in Pennsylvania, the president both attacked Biden as "dumb" before immediately lauding him as an experienced debater. "He’s a dumb guy. Always known as a dumb guy. But we look forward to seeing him in the debate. He's got a lot more experience. He's got 47 years. I’ve got 3 1/2 years. So we'll see. But he's got 47 years of experience."

Biden has brushed off Trump's onslaught of attacks on his mental fitness. "Watch me, Mr. President. Watch me. Look at us both, what we say, what we do, what we control, what we know, what kind of shape we're in," the former vice president said when asked by ABC News about the president's attacks on his cognitive ability.

While some advisers tell ABC News they're nervous as the president enters his first debate since his encounters with Hillary Clinton four years ago -- with Biden having participated in nearly a dozen debates leading up to clinching the Democratic nomination -- others are anticipating a knockout performance by the president and expect the debate to help highlight the former vice president's gaffes, sources familiar said.

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

The debate between President Trump and Joe 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, according to the Commission on Presidential Debates, with topics decided by the moderator.

Topics for Tuesday night's debate are:

ABC News' Kendall Karson contributed to this report

Viewers tuning in Tuesday night may anticipate moderator Chris Wallace to fact check the candidates in real time, but the Commission on Presidential Debates (CPD) attempted to manage those expectations over the weekend.

"There's a vast difference between being a moderator in a debate and being a reporter who is interviewing someone," CPD co-chair Frank J. Fahrenkopf Jr. said Sunday on CNN. "We don't expect Chris or our other moderators to be fact checkers. The minute the TV is off, there are going to be plenty of fact checkers in every newspaper and every television station in the world. That's not the role, the main role, of our moderators."

Wallace -- who received widespread praise for moderating the third and final 2016 debate between Trump and Clinton -- also said Sunday on Fox News that his job as a moderator is "to be as invisible as possible."

"I'm trying to get them to engage, to focus on the key issues, to give people at home a sense of, 'why I want to vote for one versus the other,'" Wallace said.

ABC will offer three hours of primetime coverage for the first presidential debate of 2020 on Tuesday evening with ABC News Chief Anchor George Stephanopoulos leading coverage from New York City joined by “World News Tonight” Anchor David Muir and “ABC News Live Prime” Anchor Linsey Davis.

A one-hour special, "Trump vs. Biden: The Main Event - A Special Edition of 20/20,” kicks off network coverage at 8 p.m. ET. The program will set the stage for the critical night for both candidates, reporting on the latest developments from the campaign trail, the stakes for both campaigns and the most important issues to voters across the country.

Pre-show coverage will begin at 7 p.m. on ABC News Live.

BIDEN'S CLAIM: Biden says 100 million Americans have pre-existing conditions, Trump says he's wrong

FACT CHECK: 100 million or more people have pre-existing conditions in the U.S.

Biden said during the debate that "100 million people" have pre-existing conditions, while Trump said that number was "totally wrong."

"There aren't 100 million people with pre-existing conditions," Trump said. The two candidates were debating about the path forward for health care in America.

Biden, who was arguing that Trump and Republicans would kick people with pre-existing conditions off of health care if they rescinded the Affordable Care Act, is correct that somewhere around 100 million Americans have pre-existing conditions, according to studies.

One study by the Health and Human Services Department in 2017 found that a large percentage of non-elderly Americans have pre-existing health conditions, ranging from 61 million to 133 million.

The study found that "at least 23 percent of Americans (61 million people) using a narrow definition based on eligibility criteria for pre-ACA state high-risk pools, or as many as 51 percent (133 million people) using a broader definition closer to the underwriting criteria used by insurers prior to the ACA" could have been denied health care or offered it at a high price before the Affordable Care Act passed because of pre-existing conditions. The study was published under the Obama administration.

There is a difference, however, between the absolute number of people with pre-existing conditions and the number of people who would be denied coverage under the Affordable Care Act. Experts point out that 54 million non-elderly Americans would've been denied insurance before the ACA, but confirmed that 100 million or more Americans have pre-existing conditions.

"54 million non-elderly adults have a pre-existing condition that would have led to a denial of insurance before the ACA. 100 million or more have a pre-existing condition that would have led to higher premiums or limited benefits," tweeted Larry Levitt, executive vice president for health policy at the Kaiser Family Foundation, referring to a study by the organization.