While President Donald Trump shuffles his campaign staff and more Republicans grumble that he has failed to formulate a clear message and stick to it, a wide array of public and private polls signal that he is facing a much more fundamental problem: A preponderant majority of Americans believe he has failed on the country's biggest challenges, primarily the coronavirus outbreak.

And history underscores the risks for Trump if he cannot shake the negative judgments. While relatively few first-term US presidents have not been reelected, every elected incumbent who has lost since the 20th century began -- a list that includes William Howard Taft, Herbert Hoover, Jimmy Carter and George H.W. Bush -- lost big. And their resounding defeats were rooted in voters' convictions that those presidents had failed on the biggest challenges facing America at the time.

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Especially ominous for Trump is the finding in multiple polls that presumptive Democratic nominee Joe Biden has opened a double-digit lead in the race even though at least as many voters view the former vice president unfavorably as favorably. The large number of voters expressing unfavorable views of Biden as Trump escalates his attacks does worry some Democrats. But the fact that the former vice president is still leading so comfortably underscores how difficult it will be for Trump to close the gap solely by amplifying doubts about Biden if he cannot improve assessments of his own performance, particularly on the pandemic.

Trump's deficit now "is not something you can tactically maneuver out of," says Liam Donovan, a Republican consultant. "There has to be some kind of fundamental inflection point that gives people hope or reassurance that some form of normalcy will return."

Put another way, Trump at this point appears to be running not so much against Biden as against the pandemic. And -- as caseloads and hospitalizations soar, the death toll ticks up and the economy remains in turmoil -- the pandemic is decisively winning that confrontation.

Trump advisers, like aides to every incumbent president facing widespread public discontent, insist they will make the reelection campaign "a choice, not a referendum" by shifting attention from their own performance to the fitness of their opponent. In practice, for incumbents in both parties, that has proved exceedingly difficult to do.

In races involving a president seeking reelection, polls consistently show that the most powerful predictor is voters' assessment of that president's performance. Both the Edison Research exit polls conducted for a consortium of media organizations and the University of Michigan's longer-running American National Election Studies underscore the same clear pattern: No matter the virtues or defects of their opponents, incumbent presidents win the vast majority of voters who approve of their job performances and lose the vast majority of those who disapprove, especially those who

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