WASHINGTON - With three weeks left until election day, President Donald Trump has not recovered from the selfinflicted wounds of his first debate with Joe Biden and, instead, has sunk further behind his challenger, a new USC Dornsife poll shows.

The latest data show the unusual extent to which the Sept. 29 debate continues to shape the campaign’s final stretch.

The encounter in Cleveland, dominated by Trump’s repeated interruptions and his cryptic statement that seemingly welcomed a right-wing extremist group, appears to be the exception to the usual rule that the impact of debates fades quickly.

The damage the debate did to Trump’s standing has persisted through his bout with COVID-19, leaving him with a deep deficit and little time to recover. (Trump’s refusal to participate in an Oct. 15 virtual debate led to its cancellation; the final debate is set for Oct. 22.)

Before the September debate, voters had relatively tepid expectations for how either candidate would do. Trump significantly underperformed those.

On a 0-to-100 scale, Trump’s performance in the debate came in 18 points below what voters had expected, a comparison of those surveyed before and since the debate shows. Independents who lean toward the GOP - a key bloc of potential swing voters - reported the most disappointment. Their rating of Trump’s performance came in 24 points below their pre-debate expectations, the poll found.

Biden’s performance was closer to expectations.

Trump also lost ground, and Biden gained, on the question of which candidate is more mentally fit for the presidency. The for mer vice president’s advantage on that question grew from 12 points before the debate to 19 points since then.

Since the debate, roughly half the voters polled said they do not believe Trump is mentally fit.

Voters’ view of both Biden’s mental and physical fitness improved after the debate - something Democratic operatives had hoped to see after Trump, 74, and his allies spent much of the spring and summer pushing to portray the 77-year-old former vice president as doddering.

The overall impact has been to swell Biden’s lead. The USC Dornsife poll tracks the race each day, using a 14-day rolling average. As predebate interviews have cycled out of that average, Biden’s lead has climbed steadily from 9 points on the day of the debate to 13 points as of Tuesday - 54% to 41%.

Other recent nationwide surveys have shown similar results, with Biden holding a 12-point lead, 54% to 42%, among likely voters in a Washington Post/ABC poll released Monday, 10 points in the most recent Fox News survey, and 10 points in a survey of more than 10,000 registered voters by the Pew Research Center.

Biden similarly leads in most polls of battleground states. The Trump campaign has pulled back on advertising in several states, including Wisconsin and Michigan, which were key to his 2016 victory, and the competition is now taking place most intensely in three states that Trump won four years ago, Pennsylvania, Florida and Arizona.

It’s still possible for Trump to engineer a comeback now that he’s returned to the campaign trail: The history of presidential elections over the last four decades includes a couple of contests in which shifts of 7 percentage points took place late in the campaign. But Trump would need a bigger shift than that to get within striking distance of an electoral college majority, and there’s no question he finds himself in a deep hole with time - and campaign cash - running short and millions of voters already casting ballots.

Biden’s current lead is nearly twice as large as the 7-point advantage Hillary Clinton had in national polling averages three weeks before the 2016 election.

Several other factors distinguish Biden’s lead from Clinton’s:

- Clinton never hit 50% in averages of polls taken after she and Trump clinched their respective nominations, making her vulnerable to a late shift by undecided voters. Biden has stood at or above 50% in polling averages consistently since June.

- The Clinton-Trump race featured constant swings in the candidates’ standings; the current race, by contrast, has set records for stability.

- And four years ago, voters had strongly negative impressions of both Trump and Clinton. This year, voters have a negative impression of Trump, 60% to 39%, but not of Biden. Voters divide closely on him, but his overall standing is positive: 51% favorable, 47% unfavorable.

- By contrast with the “hold your nose and vote” image of 2016, fewer voters now report themselves undecided - only 4% - and fewer intend to vote for third-party candidates.

“The debate was really bad for Trump,” said Bob Shrum, the veteran Democratic strategist who directs USC’s Center for the Political Future, which co-sponsors the poll. To the extent that there’s good news for the president, it’s that his standing may have hit bottom, Shrum said. “I don’t know how much more vote is available” for Biden.