The Battlegrounds Within Battlegrounds

Let’s start with Florida, a diverse but conservative-leaning state that is almost always close in presidential elections. It’s likely this year’s race will again be decided by a percentage point or two.

Miami-Dade County, Fla. A Democratic stronghold, it is not a county Mr. Trump would hope to win. But this majority-Hispanic county was a disappointment for Democrats in 2018, especially in heavily Cuban-American precincts. Younger Cuban voters have started identifying as Trump Republican here.

Pinellas County, Fla. Perhaps the biggest swing county in the state, which backed Mr. Trump after twice backing Barack Obama, it is a Florida microcosm: solid Democrats in St. Petersburg and Midwestern retirees elsewhere.

Osceola County, Fla. Part of the greater Orlando area, it is increasingly Hispanic. Conservative retirees have been joined by hundreds of thousands of Puerto Ricans, who did not register in expected numbers to give Democrats an advantage in 2018, and so far, are lagging behind other groups in early voting.

Let’s look at some critical counties in North Carolina. The state has cities with large communities of Black voters, moderate professionals and college students and also big stretches that are more rural, whiter and conservative.

Union County, N.C. In 2016, Mr. Trump easily won this suburban Republican bastion near Charlotte. Republicans remain dominant, but signs of disaffection with the president, along with an upswing in “unaffiliated” voters, give Democrats hope they can trim Mr. Trump’s margin.

Wake County, N.C. One of the nation’s fastest-growing counties, Wake has shifted steadily leftward over the past 20 years, supporting Hillary Clinton in 2016 by more than 100,000 votes. An influx of out-of-staters since then stands to boost the Democrats even more, potentially offsetting high Republican numbers in rural areas.

Robeson County, N.C. A former Democratic stronghold, this economically depressed county went for Mr. Trump in 2016. The prize will likely go to the candidate most popular among the Lumbee Indians, the county’s largest group. Mr. Trump held a rally here in October, and both campaigns pledged to support the tribe’s quest for federal recognition.

Let’s move on to Pennsylvania, which has two huge Democratic cities, big swaths of formerly Republican suburbs and a deeply conservative rural middle. This year’s election may hinge on this state, one of three that Mr. Trump won by less than one percentage point in 2016.

Westmoreland County, Pa. Typical of other counties where Mr. Trump outperformed with white working-class voters four years ago, this area near Pittsburgh is where he must win even bigger margins to counter a likely Democratic surge in the suburbs.

Chester County, Pa. Democrats must continue their 2018 midterm surge in this suburban Philadelphia county, especially with college-educated women, or Mr. Trump could carry Pennsylvania again.

Erie County, Pa. One of three counties in the state that Mr. Trump flipped in 2016, its mix of a working-class post-industrial economy and rural towns makes it “the oracle of Pennsylvania,” in the words of a Democratic strategist.

Philadelphia County, Pa. The big question here is whether Mr. Biden can re-energize Black voters — Democrats’ core supporters — after Hillary Clinton’s lackluster showing in 2016. Mr. Biden will have to boost the numbers to counter Mr. Trump’s margins with rural white voters. The Trump campaign has taken on aggressive tactics, like videotaping voters at ballot drop boxes.

Michigan, historically Democratic, provided one of Mr. Trump’s most surprising victories in 2016. He won the state by 0.3 points.

Macomb County, Mich. Heavily unionized and mostly white, the state’s third largest county has picked the statewide winner in the last seven elections for governor and president.

Oakland County, Mich. Once solidly Republican, it is a more affluent neighbor of Macomb County and has been trending Democratic. It is a prime example of the changes that are taking place in many of the nation’s suburbs. In 2018, it gave Gov. Gretchen Whitmer the biggest margin for a Democrat in 20 years.

Kent County, Mich. This traditional Republican stronghold — home to Grand Rapids, where President Gerald Ford was raised — has moved away from the Republican Party in the Trump era.

In Wisconsin, the state’s politics are shaped by the liberal cities of Milwaukee and Madison; the rural north and west of the state; and the affluent and predominantly white suburbs that lie in between.

Brown County, Wis. Among the top counties that will decide the state’s winner is the home of vote-rich Green Bay. It’s a swing county that in 2018 voted for the Republican candidate for governor, Scott Walker, and the Democrat for Senate, Tammy Baldwin. Mr. Trump won blowout margins here compared with Mitt Romney in 2012.

Waukesha County, Wis. It is the largest of Milwaukee’s suburban counties. Long a Republican stronghold, the county underperformed for Mr. Trump in 2016. Mr. Biden has forged inroads here, but it’s not clear how deep they are.

Dane County, Wis. This is home to the University of Wisconsin-Madison, and it’s where Democrats surged in an April 2020 race for the State Supreme Court. Nearly as many votes were cast here as in Milwaukee County, even though Dane has less than 60 percent of Milwaukee’s population. Heavy turnout in early voting suggests Mr. Biden is claiming those votes.

Grant County, Wis. Emblematic of southwest Wisconsin, it is one of the state’s swingiest regions, where weak partisan identity saw voters shift from Mr. Obama to Mr. Trump.

One of the fastest-changing states on the electoral map, Arizona has gone from being a Republican stronghold to a true battleground.

Maricopa County, Ariz. Home to Phoenix and more than 60 percent of the state’s electorate, it is Arizona’s most important county. It went narrowly for Mr. Trump in 2016, but two years later supported a Democrat, Kyrsten Sinema, for senator. The question is whether the county’s changing demographics will tip the state to a Democratic president for the first time since 1996.

Pima County, Ariz. The home of Tucson, Democrats typically run up the score here.

Pinal County, Ariz. The state’s third-largest county is a Republican redoubt. Mr. Trump will have to turn out enough rural white voters to help protect the 3.5-point margin he won the state with in 2016.