1st presidential debate between Trump and Biden spirals into chaotic clash

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:

• Trump and Biden records
• The Supreme Court
• COVID-19
• Economy
• Race and violence
• Integrity of election

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.