A Trump-Supporting Sheriff’s Stand Against Kari Lake in Arizona

Mark Lamb, the Pinal County sheriff and a Republican candidate for U.S. Senate, strode around his spacious office overlooking the Sonoran Desert, proudly pointing out his accumulated memorabilia.

Here was the old cowboy rifle his wife had gifted him when he first won his post in 2016. It sat across from an animal skull and an engraved pistol, and it was below a framed New York Post cover story praising him as the “last line of defense” against drug traffickers. There was a signed card from the Trumps — Donald, Melania and Barron — thanking him for attending a Christmas party at Mar-a-Lago, the former president’s private home and resort. American flags and eagles abounded.

“I’m a dadgum patriot,” Mr. Lamb said.

But he’s not Kari Lake, the former news anchor and close Trump ally who has dominated a race Mr. Lamb says he’s still hopeful he can win.

For months Ms. Lake, 54, has held substantial leads over Mr. Lamb, 52, in surveys. With Mr. Trump’s endorsement and the backing of the Washington Republican establishment — and all the advertising dollars that came with it — she is widely expected to sail to victory in the primary on Tuesday night. The winner will go up against Representative Ruben Gallego, who is running unopposed on the Democratic side, for the seat being vacated by Senator Kyrsten Sinema, the Democrat-turned-independent who decided not to run for re-election.

Mr. Lamb’s struggles to break through are reflective of the broader party dynamic, Arizona political operatives and strategists said. Despite burnishing his brand as a cowboy hat-wearing, Americana-loving, immigration hard-liner, a résumé that might normally appeal to the Republican faithful in a border state, it is Ms. Lake who is near-ubiquitous, thanks to