"We built the greatest economy in the world; I'll do it a second time," Mr. Trump said earlier this month, road-testing a theme he will deploy in the coming weeks.
Still, a recent wave of polling has fueled Republican anxieties, as Mr. Biden leads in virtually every competitive state.
The surveys also showed Republican senators in Arizona, Colorado, North Carolina and Maine trailing or locked in a dead heat with potential Democratic rivals " in part because their fate is linked to Mr. Trump's job performance. If incumbents in those states lose, and Republicans pick up only the Senate seat in Alabama, Democrats would take control of the chamber should Mr. Biden win the presidency.
"He's got to run very close for us to keep the Senate," Charles R. Black Jr., a veteran Republican consultant, said of Mr. Trump. "I've always thought we were favored to, but I can't say that now with all these cards up in the air."
Republicans were taken aback this past week by the results of a 17-state survey commissioned by the Republican National Committee. It found the president struggling in the Electoral College battlegrounds and likely to lose without signs of an economic rebound this fall, according to a party strategist outside the R.N.C. who is familiar with the poll's results.
The Trump campaign's own surveys have also shown an erosion of support, according to four people familiar with the data, as the coronavirus remains the No. 1 issue worrying voters.
I'm thinking this will all come down to a second wave. Should it arrive and be even similar of a disruption, Trump and the Republican Senate may just be sunk. I never though it would happen. It fits with the information that I gathered at an regional economic round table that was held mere weeks before this pandemic. The Dean of Economics at the University of San Diego declared the fears of a recession looming were overblown, that our economic outlook continued to look nice and rosy, which was a bit surprising to him. Of course, he left us with a disclaimer to that rosy outlook: it could very well implode should a major event take place. When asked for an example of such an event, the dean provided two: a war or a pandemic. To this end as it relates to Trump's electability as an incumbent, I didn't think anything would be capable of knocking him off his pedestal. Clearly a war just might be the thing to turn his current electability around, but when the dean brought up a pandemic, I then thought it would probably be the ONLY thing that would have the effect significant enough to sway voters from reelecting a modern U.S. President incumbent. That would be unprecedented.