However, the pollster rejected the idea that “shy” Trump voters, those who support him privately rather than vocally, will swing the election for the former Republican president.

Instead, he said that pollsters failed to reach Trump voters and therefore underestimated the true extent of his support.

“The likely problem is what pollsters call non-response bias. It's not that Trump voters lie to pollsters; It’s because pollsters didn’t get enough of them in 2016 and 2020,” he said.

Democratic campaign strategists now privately admit that Ms. Harris is losing ground to Trump in the polls ahead of Election Day, particularly in Wisconsin and Michigan.

“Everyone always says, 'It's close.' Yes, it's close, but are things going our way? No. And no one wants to openly admit that,” one Democratic strategist told The Hill.

They continued: “Could we still win? Perhaps. Should anyone be even a little optimistic right now? NO.”

Mr. Silver said the vice president, who is running as the first female president, risks running afoul of the “Hillary effect.” “The only time a woman was nominated as her party's candidate, undecided voters were heavily against her. So perhaps Ms. Harris should be concerned about a “Hillary effect,” he said.

Although polls showed the two candidates neck and neck, Mr. Silver encouraged Americans to prepare to be wrong.

“According to my model, there is about a 60 percent chance that a candidate will win at least six of seven contested states,” he said.