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.

Wallace's first question to both candidates was on the Supreme Court. He asked why Trump believes he is right in confirming a justice before the election and why Biden believes he is right in saying the decision should wait until after Election Day.

"I will tell you very simply, we won the election. Elections have consequences," Trump said. "We have the Senate, we have the White House and we have a phenomenal nominee respected by all top top academic good in every way, good in every way."

"And by the way, the Democrats, they wouldn't even think about not doing it if they have -- the only difference is to try and do it faster. There's no way they would give it up. They had Merrick Garland, but the problem is they didn't have the election so they were stopped."

When the question was given to Biden, he said that the American people have a right to say who the Supreme Court nominee should be and that that nomination should wait until the November election concludes as Americans have already begun voting.

"It's just not appropriate to do this before this election," Biden said. "Roe v. Wade and the Affordable Care Act could both be struck down by a conservative Supreme Court."

Multiple Senate Democrats are refusing to meet with Trump's nominee, Amy Coney Barrett, citing the impending election, and have expressed mounting concern with her stance on health care as the Supreme Court is set to hear arguments on the fate of the Affordable Care Act one week after the election.

Senate Republicans, meanwhile, are moving ahead with the nomination despite claims they made in 2016 about not confirming Garland during an election year.