The one-two punch of the president’s coronavirus diagnosis and his widely panned debate performance also has Republicans worried they could lose control of the Senate.
With little more than three weeks until Election Day, Senate races are competitive in some reliably red states, including South Carolina and Kansas, aided by a surge in Democratic fundraising that has put both the Republican Party and Mr Trump’s own campaign at an unexpected financial disadvantage.
The president will aim for a reset this week, hoping an aggressive travel schedule and Judge Amy Coney Barrett’s Supreme Court confirmation hearings will energise his most loyal supporters and divert attention from a virus that has killed more than 214,000 Americans on his watch.
Optimists in Mr Trump’s inner circle point to his unique ability to command attention and to his 2016 campaign, which also seemed destined for defeat before a late shift.
But that comeback was aided by outside forces against an unpopular opponent. This year’s campaign, other Republicans worry, may instead resemble 1980 or 2008, which appeared to be close races until, at the end, they were not.
“It’s not good for my side,” said veteran Republican pollster Whit Ayres. “Pretty obviously, in many ways down-ballot Republicans are in the boat with Donald Trump. That’s good for Republicans in deep red states, but more problematic for those in swing states.”
Asked for any bright spots for the Republican field, Mr Ayres said: “I’m wracking my brain and just struggling.”
This account of Mr Trump’s re-election effort was compiled from interviews with nearly two dozen White House and campaign officials and Republicans close to the West Wing, many of whom were not authorised to publicly discuss private conversations. It describes how a campaign that was generally stable for months is enduring a series of historic jolts.
Republicans began sensing warning signs last month.
The death of Supreme Court Justice Ruth Bader Ginsburg animated conservatives but also electrified Democrats, driving a tsunami of cash toward Mr Biden and down-ballot Democrats. Mr Trump’s heavily derided debate performance only exacerbated the problem, as even his own supporters found him rude as he so frequently interrupted Mr Biden.
Reviewing data afterwards, campaign aides worried as they started to see Mr Trump’s support begin to slip. The president’s coronavirus diagnosis only compounded the problem, particularly with older voters.
“It’s not good,” said Alex Conant, a senior campaign adviser to Senator Marco Rubio’s 2016 presidential campaign. “It’s been a long time since Donald Trump has had any good news, and when he does have good news, he manages to step on it.”
The Trump campaign insists the president can win re-election, saying his return to the road will excite his base while claiming that public polling has undercounted their supporters.
But national polls have shown Mr Biden with a significant lead. And while the margins in the battleground states are smaller, Mr Trump has faced stubborn deficits in most of the states that will decide the election.
Republican Senator Ted Cruz said on Friday: “I think we could lose the White House and both houses of Congress, that it could be a bloodbath of Watergate proportions.” That was a reference to the two national elections after Republican President Richard Nixon resigned in 1974, in which Democrats significantly increased their congressional majorities and took back the White House.
Advisers privately concede Mr Trump, who has not moderated his tone on the virus after falling ill, has little chance of victory without Florida, which will be the site of his first post-diagnosis rally on Monday. And a loss in Arizona would require him to hold onto Pennsylvania.
“He’s in trouble, there’s no question. By every traditional measuring stick, this looks like a Biden landslide,” said Ari Fleischer, former press secretary for Republican president George W Bush. “It’s hard to predict what will happen in Congress, but anytime there is a large-scale victory, it has the potential to create tides to sweep people out.”
The fate of the Senate has increasingly weighed on Republicans, who see tough races in Maine, Colorado, Arizona and North Carolina and even worries in deep-red Kansas and South Carolina.
Some Trump backers are warning GOP Senate candidates are now at an inflection point and some, including senators John Cornyn of Texas and Martha McSally of Arizona, took small steps to distance themselves from the president in recent days.
One significant opportunity to potentially change the trajectory of the race is Ms Barrett’s Senate confirmation hearings this week. Mr Trump’s campaign believes the hearings could change the political narrative away from the virus and draw attention to Mr Biden’s refusal to say whether he would expand the Supreme Court.