WASHINGTON - The first presidential debate between Joe Biden and President Donald Trump has a strong chance of being the most-watched political event in U.S. history, and the enormous potential audience on Tuesday is just one factor that has heightened the stakes for both candidates.
For Trump, stuck for months on a losing trajectory, the debate stands as one of the few remaining opportunities to shift how Americans view the election and to reach voters beyond his deeply committed core of supporters.
For Biden, who has maintained a significant, but not unbeatable, lead nationally and in crucial swing states, the encounter provides a chance to bolster his standing with a key slice of the electorate - voters who have turned against the president but remain unconvinced about his challenger.
With that opportunity, however, comes risk for the former vice president, who, in debates during the primary elections, sometimes appeared to lack energy or focus.
Debates are often overrated as a turning point in campaigns, said Mike Murphy, the veteran Republican strategist and fervent Trump critic who co-directs USC's Center for the Political Future. Research by political scientists shows that for all the attention they get, general-election debates only rarely have an impact that lasts more than a week or so.
"This debate is a little different," Murphy said, in part because Trump and his campaign have worked furiously to raise doubts in voters' minds about Biden's mental fitness and physical stamina.
"For Biden to break through and show he's sharp and on top of it, that's an opportunity but also a risk," Murphy said. "If Biden has a bad debate, Trump has a whole month to exploit it."
Democratic pollster Anna Greenberg agreed. "Republicans have been very effective" in spreading the idea that Biden is physically or mentally impaired, she said. "You hear a lot of it" in focus groups of voters.
Since most swing voters don't pay much attention to political news, the debate "will probably be the first opportunity that a lot of voters have to see Biden and see that he's not impaired," she said. "That's the main thing."
Indeed, many Republicans worry that Trump, with his frequent references to Biden as "sleepy" or "out of it," has "lowered the bar" too much for what voters may expect from the Democratic challenger, said Republican strategist Alex Conant, a former top aide to Sen. Marco Rubio of Florida.