A few years ago, a flood of good jobs into the suburbs around Phoenix might have been great news for Republicans, bringing an influx of middle-class and predominantly white voters to a county that accounts for three of every five votes cast in Arizona.

But particularly under Mr. Trump, the suburban political calculus has changed. Voters in the suburbs are now far less likely to support him or members of his party than they were just five years ago.

“It used to be that in Maricopa County, if you put an ‘R’ in front of your name, you’d win,” Chuck Coughlin, a longtime Republican strategist based in Phoenix, said in an interview. Now, he added, “that is not the case.”

In the Times/Siena poll, Mr. Biden trounced Mr. Trump by 58 to 33 percent among likely voters in Phoenix. But he was also running even with the president in the rest of Maricopa County, with each candidate receiving 45 percent support.