Representative Ruben Gallego, a Phoenix-area Democrat and military veteran, has won the Senate race in Arizona, The Associated Press said on Monday night — a bright spot for Democrats after Republicans regained control of the chamber.

Mr. Gallego defeated his Republican opponent, Kari Lake, a fiery ally of President-elect Donald J. Trump’s, after leading her by sizable margins throughout the summer and the fall. The final result was much closer than some predictions, another sign of Republican strength throughout the country.

Mr. Gallego is a five-term congressman who grew up in a working-class Chicago family, working jobs as a teenager to support his family and sometimes sleeping on the floor of their apartment. He attended Harvard University and fought in the Iraq war as a Marine. He will become Arizona’s first Latino senator, replacing Senator Kyrsten Sinema, an independent and former Democrat.

At a news conference in Phoenix late Monday night, Mr. Gallego, with tears in his eyes, referred to his humble roots.

“For the kids sleeping on the floor, dreaming about a better, better America and a better, better future, this victory is for you,” he said.

After going decades without electing a Democrat to the Senate, Arizona’s voters have now done so in four successive elections, underscoring the state’s shift from reliable conservative stronghold to competitive battleground. Voters backed Ms. Sinema in 2018, picked Senator Mark Kelly in a special election in 2020 and then elected him again in 2022.

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