Mr. Biden leads Mr. Trump 49 percent to 41 percent in Arizona, with just 6 percent of likely voters saying they were undecided, according to the survey, which was taken before and after the president announced that he had tested positive for the virus that causes Covid-19 but after his caustic debate performance last week. The poll has a margin of error of 4.2 percentage points.

The results are essentially unchanged from a Times-Siena poll of the state last month, which found Mr. Biden leading 49 percent to 40 percent. The poll illustrates the depth of Mr. Trump’s difficulties in Arizona — an incumbent Republican president trailing by a significant margin a month before an election in which most voters have made up their minds…

In a sign of a brewing suburban backlash against the president, Mr. Biden’s lead in Maricopa County, the Phoenix-anchored population hub of the state, matches his statewide advantage. He’s leading by nine points in the county, which accounts for over 60 percent of the state’s population. It’s highly difficult to win Arizona without winning Maricopa, which Mr. Trump captured by three points in 2016.