Democratic Rep. Ruben Gallego has the upper hand over Republican Kari Lake in Arizona’s U.S. Senate race, according to a new survey.

A Noble Predictive Insights poll released Thursday found Mr. Gallego, a former Marine who has served in Congress since 2015, leads Ms. Lake, a former newscaster who lost her 2022 bid for governor, 47% to 40%. His lead has shrunk by 3 percentage points since May.

Mr. Gallego has done a better job of solidifying his support among Democrats than Ms. Lake has among Republicans and has a 46% to 29% lead among independents, according to the poll.

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“Gallego is in a better position in this race. In our polarized world, it’s hard to imagine things going much better for him than they have in this past year,” said Mike Noble, founder of the poll. “Lake clearly has room to grow with wayward Republicans and swing voters. The real question — is she the sort of candidate that can grow her support?”

Democrats are fighting this fall to retain their slim Senate majority.

Sen. Kyrsten Sinema is vacating the seat in Arizona. Ms. Sinema passed on running for reelection after ditching the Democratic Party and becoming an independent.

Ms. Lake, meanwhile, has hitched her wagon to former President Donald Trump, who is locked in a competitive battle with Vice President Kamala Harris in Arizona. Mr. Trump lost Arizona to President Biden by less than 11,000 votes in 2020.

Ms. Lake has some work to do on her image after running a no-holds-barred bid for governor that turned off some Republican voters and independents.

Half of registered voters view her in an unfavorable light, and 37% have a positive view of her.

The poll of 1,003 registered voters in Arizona was conducted Aug. 12-16, and has a margin of error of plus or minus 3 percentage points.