Representative Ruben Gallego, running unopposed in the Democratic primary for U.S. Senate in Arizona, placed a reservation on Monday for $19 million in television advertisements for the general election this fall, according to his campaign — the largest any Senate candidate in the country has made so far.

Come fall, Mr. Gallego is likely to be locked in a tight race with Kari Lake, the Republican and ally of Donald J. Trump. They are set to be competing for the seat now held by Senator Kyrsten Sinema, the Democrat turned independent who decided not to run for re-election. Ms. Lake faces a Republican challenger but is expected to easily prevail in the Republican primary on July 30.

The contest is rated a “tossup” by the nonpartisan Cook Political Report, but Ms. Lake may enter it as an underdog if she and her allied groups are unable to keep up on the airwaves with Mr. Gallego, who is raising and spending money at a torrid pace. Mr. Gallego’s $19 million reservation is expected to go toward ads that will run in the Phoenix and Tucson markets, beginning on June 18, though it does not commit the campaign to following through with the purchase.

The reservation is greater than those placed so far by incumbent Democrats facing re-election battles; Senator Sherrod Brown of Ohio has reserved $14.3 million; Senator Jacky Rosen of Nevada has reserved $13.9 million; and Senator Jon Tester of Montana has reserved $8.9 million, according to the media-tracking firm AdImpact.

Ms. Lake, who made bogus claims of election fraud a key aspect of her failed bid for governor in 2022, has worked to broaden her base of support this year in her second statewide run by courting establishment Republicans. She has earned the endorsement of the National Republican Senatorial Committee, which is the Senate Republicans’ campaign arm and which is backing a new television ad debuting on Tuesday, and she has raised campaign money alongside top Senate Republicans.

But she has struggled to moderate her tone, and some national Republicans have indicated that, before deciding whether to invest heavily on trying to capture Arizona, they are prioritizing what they see as more winnable Senate races as they work to take back control of the chamber.

Mr. Gallego, meanwhile, can count on $23 million reserved for advertising by the Senate Majority PAC, a Democratic group, and he has out-raised and outspent Ms. Lake so far in the race. A member of Congress who represents a district in Phoenix, Mr. Gallego has long described himself as a progressive but has shed that label as he courts a broader statewide electorate. His ability to flood the airwaves with positive advertisements could help him ensure that most voters’ first impression of him is as a pragmatic lawmaker and former Marine.