Nevada has been hit hard by the coronavirus pandemic, which could be a wild card in the election. There have been more than 96,000 cases in the state so far, and more than 1,750 deaths, according to a New York Times database. The pandemic has hammered the state’s economy, which relies heavily on the tourism industry, causing unemployment to soar to one of the highest rates in the country; in September, it stood at about 13 percent, disproportionately affecting Latinos and working-class union voters, who are a large part of the Democratic Party’s base in the state.

Some Democratic strategists are now bracing for the possibility that a significant number of would-be Democratic voters must contend with more immediate concerns, including feeding their families, than casting a ballot.

Mr. Trump has continued to fight for Nevada, visiting the state twice since securing the Republican nomination for re-election; on Wednesday, he is planning to hold a rally just across the border in Bullhead City, Ariz. In September, The Cook Political Report shifted its assessment of the Nevada race in Mr. Trump’s direction, from “likely Democrat” to “lean Democrat.”

But Mr. Biden’s polling lead underscores the shifting dynamics of a consummate swing state that has taken on a Democratic tilt.