PHOENIX (AP) — Democratic Rep. Ruben Gallego began airing the first television ads of his U.S. Senate campaign on Tuesday as the crucial Arizona race takes shape as a one-on-one contest after incumbent Kyrsten Sinema declined to run for a second term.

Gallego's ad introduces him to voters as a U.S. Marine who served in a unit that faced especially fierce fighting in Iraq and lost 48 men.

The Arizona race is one of a handful of contests that will determine control of the Senate. It is now ramping up after Sinema, who was elected as a Democrat but left the party, made clear she would not enter the race as an independent candidate, paving the way for Democrats to line up behind Gallego.

Gallego is emphasizing his military service and financially humble upbringing as he looks to expand his stature beyond the safely Democratic district in Phoenix that he's represented since 2015.

In his ad, Gallego says he worked as a cook and a janitor growing up and got into Harvard University with no money or connections. He also makes an oblique reference to his battle with post-traumatic stress disorder, or PTSD, after returning from Iraq, saying he “survived the memories of war so I can be a good dad.”

The ad does not mention Kari Lake, the front-runner for the Republican nomination, who is especially well known after a nearly three-decade career as a local news anchor in the Phoenix market. She's also built a national profile among Republicans as a fiercely loyal supporter of former President Donald Trump despite her loss in last year's race for Arizona governor, which she continues to contest.

Lake aired an ad of her own in January during coverage of the Iowa caucuses and the New Hampshire primary.

Republicans need a net gain of one or two seats to control the Senate, depending on which party controls the White House after the November election.

Arizona has long been a Republican stronghold, but Sinema's 2018 election marked a period of ascendance for Democrats. They've now won three straight Senate races along with the state's top three offices with a coalition that includes right-leaning voters who typically vote Republican but dislike Trump.

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