Representative Ruben Gallego, the expected Democratic nominee for Senate in Arizona, raised $7.5 million in the first three months of the year, his campaign said on Tuesday, a significant sum for what is likely to be a competitive general election race in the battleground state.

The haul of campaign cash adds to a strong fund-raising cycle thus far for Mr. Gallego, a former Marine and progressive congressman whose candidacy will test how willing Arizonans are to elect a senator who is decidedly left of center. After decades of being a reliably Republican state, Arizona has elected a succession of Democrats to statewide office in recent years.

In November, Mr. Gallego, who has no major primary challengers, is likely to face Kari Lake, a former television anchor and close ally of former President Donald J. Trump. That contest will determine who will succeed Senator Kyrsten Sinema, who is not seeking re-election. Ms. Lake faces a primary challenger, Sheriff Mark Lamb of Pinal County, but leads him by a wide margin in polls.

So far, Mr. Gallego has raised more money than Ms. Lake, who has not yet released her most recent quarterly fund-raising numbers. Ms. Lake is a more recent entrant to the race than Mr. Gallego, who announced his run in January 2023. In the last quarter of 2023, his campaign raised $3.3 million, while hers netted $2.1 million.

Mr. Gallego’s most recent tally compares favorably with sums raised by Democratic incumbents such as Senator Sherrod Brown of Ohio ($6.6 million in the last three months of 2023) and Senator Jon Tester of Montana ($5.5 million in the same period), though it is difficult to compare fund-raising numbers across states.

His funding edge, if it continues, could allow him to flood the airwaves with advertising in a state that has a relatively expensive television market. Mr. Gallego’s campaign spent about $1 million on his first advertisement of the cycle, which began running in March. Ms. Lake, who also ran for governor in 2022, beat him to the airwaves, broadcasting her first ad in January.

The Gallego campaign — announcing its rosy fund-raising numbers two weeks before the required April 15 deadline — said that it had more than $9.6 million cash on hand, and that more than 100,000 people donated in the last quarter, more than half of whom were first-time donors.

“Thanks to the support of hundreds of thousands of small-dollar donors who have chipped in what they could to help elect Ruben Gallego, we are building the infrastructure to win this November,” Nichole Johnson, Mr. Gallego’s campaign manager, said in a statement.

Outside groups could help make up any deficit Ms. Lake faces in fund-raising. She has the backing of the National Republican Senatorial Committee, a group dedicated to electing Republicans to the Senate, and has raised money with top Senate Republicans like John Barrasso of Wyoming. She took in $330,000 at a fund-raiser in Washington alongside nearly 20 Senate Republicans last month, and will raise more money at Mr. Trump’s Mar-a-Lago club in Florida this week.

Mr. Gallego, too, will be buoyed by outside groups. The Senate Majority PAC, a Democratic group, announced a $239 million television reservation last month across seven states, including $23 million in Arizona.