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."
On Friday, the president delivered an extended attack on Biden, comparing the showdown to a boxing match while repeatedly pointing to his head and saying, "To win matches you need that up here. ... This wins, probably, it's 50% of it. ... It's great that it's that way, isn't it?"
"[Biden's] not exactly prime time. ... This is not prime time for Joe. He never had too much of a prime time," the president added.
It's a line of attack the president has repeatedly used against Biden, and one some of his top advisers have urged him to tone down in the lead-up to the debates, while his campaign and allies mount a last-minute attempt to raise exceptions for Biden's performance.
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.
Some advisers close to the president were pleased he took part in a town hall earlier this month with ABC News seeing it as a rare moment to interact with voters, some who challenged him, and a helpful way to prepare him for the debates.
Sources familiar with the president's planning say they expect he will be gathering with a small team of advisers this weekend to go over topics ahead of Tuesday night's faceoff.
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.
The sessions have often featured conversations about potential debate topics or issues of the day that are on the president's mind with few traditional sessions such as mock debates, according to sources familiar with the meetings. Trump has also been eager to target Biden personally, looking to lean into attacks on his son, Hunter Biden, once he's on the debate stage, including over his Ukraine business dealings while his father was in office, though he has denied any wrongdoing and has never faced charges, sources said. The president even previewed that strategy at a rally Fayetteville, North Carolina, laying into the former vice president's son and saying, "I think it'll be brought up in the debate."
When asked on Saturday by reporters if he plans to go on the attack Tuesday night, the president said, "Well, I can't tell you what I'm going to do," before adding, "I mean, I don't know ... I have no idea."
The Trump campaign did not respond to a request for comment.
In the lead-up to the first debate, some advisers have pushed for the president to talk up Biden as a seasoned debater, who will enter with decades of experience given his time in Washington, sources familiar with the president's debate preparation told ABC News.
And while the president has at times made that point, far more often he opts to hammer his opponent as mentally unfit.
"We have a debate coming up and who knows ... you know what, he's been doing it for 47 years. I have been doing it for 3 1/2 years so he should be able to beat me, I would think. He's much more experienced. He's great," Trump said at a campaign event in Dayton, Ohio, on Monday.
Minutes later at that same Ohio event, the president said of Biden: "The guy can't speak without the teleprompter."
The setup for the first debate is shaping up to be unique to the times, with both campaigns agreeing to no formal spin room following the event and only a limited in-person audience.
The president enters the debate trailing Biden nationally by over seven points, according to FiveThirtyEight's average. And recent ABC News/Washington Post polls show the president is trailing in key battleground states among likely voters, including Wisconsin, where he trails Biden by six points, and Minnesota, where he's down by 16 points. But in Florida and Arizona, two states that will be crucial to picking the next president, the contests remain closely divided, with registered voters in Florida split almost evenly for the president, 47%-48%, while in Arizona the race stands at 47%-49% among registered voters.
On top of the president's repeated attacks on Biden's mental fitness, the Trump campaign has centered its strategy around trying to convince voters that Biden is mentally unfit for office, spending millions on ads that portray him struggling to speak while arguing he would be a puppet for the "radical left."
Back in June, the Trump campaign launched a brutal television ad titled "Fortitude," that worked to paint a dire picture of Biden's mental state, putting nearly $6 million behind the ad that features a narrator claiming the former vice president is "slipping" and "now at the age of 77 years old and running for president for the third time, Biden is clearly diminished."
Despite this, in recent weeks the Trump campaign has tried to boost expectations for Biden's debate performance.
"We have to be prepared for the tuned-in Joe Biden," Tim Murtaugh, communications director for the Trump campaign, said in an interview with Fox News, noting the former vice president "has been a Washington politician for 47 years" and "spent decades in the Senate where all they do is debate."
Miller, a Trump campaign senior adviser who had been arguing early that Biden may do well in a debate setting, told The Washington Post that in debates the former vice president "doesn't have as many gaffes as he does in his everyday interviews."
"I would make the argument that Joe Biden would even be the favorite in the debates since he's been doing them for 47 years," Miller said.
Trump has also started injecting wild claims into the mix, baselessly accusing Biden of taking "drugs" before his one solo debate with Sen. Bernie Sanders during the Democratic primary earlier this year.
The president tried the same tactic during the 2016 election when he suggested Clinton "was all pumped up" following the first debate and called for drug tests before the next one.