When Trisha and Andy Allcock decided to leave their native Ohio, they set their sights on Wisconsin.

Madison, the second-largest city in the state, and its suburbs in Dane County checked all of the boxes: job opportunities, excellent public schools for the couple’s two children, and hiking and camping within easy reach.

Another irresistible selling point? The dark-blue liberal politics of Dane County.

“We’re very extreme Democrats,” said Ms. Allcock, 43, a marketing executive who said she “absolutely” planned to cast a ballot for Vice President Kamala Harris.

Voters like Ms. Allcock could decide the presidential election in Wisconsin, which flips back and forth between Republicans and Democrats in statewide contests by the slimmest of margins.

Dane County, where Ms. Harris is holding a rally on Friday night, is home to the state government, the sprawling flagship University of Wisconsin campus and, particularly in the last decade, a flourishing health care and technology industry.

It is also the fastest-growing county in the state, changing what it takes for presidential candidates to win a statewide race in Wisconsin. In recent years, Dane County has seen a rapid population rise in both the city of Madison and the suburbs that are suddenly blossoming around it, fueled by growth at the university and the local economy.