Nevada G.O.P. Senate primary heats up as the long shot goes after the front-runner.

The Army veteran Sam Brown remains the favorite, but the wealthy former ambassador Jeff Gunter, with a pro-Trump message, is trying to shake up the race.

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Sam Brown quickly consolidated support after entering the Nevada primary last July.

Nevada’s once-sleepy Republican primary for Senate, which has been dominated by Sam Brown, a U.S. Army veteran, was jolted to life in the past week, when a deep-pocketed rival took aim at the front-runner.

Jeff Gunter, the ambassador to Iceland under former President Donald J. Trump, is unloading a $3.3 million advertising campaign with a MAGA message, according to his campaign, hoping to cut into Mr. Brown’s dominant lead over the crowded field.

A television ad from Mr. Gunter that began airing on Wednesday called Mr. Brown “the newest creature to emerge from the swamp,” tying him to Senator Mitch McConnell of Kentucky, the minority leader, and deriding his primary opponent as “Scam Brown.”

Mr. Brown, a veteran who was wounded severely in Afghanistan in 2008, consolidated party support after entering the primary last July, earned the endorsement of the National Republican Senatorial Committee and has appeared at fund-raisers around the country with prominent Republicans. He has lapped his competitors in fund-raising, pulling in $2.4 million in the most recent quarter, according to fund-raising reports.

Mr. Brown has sought to employ the Trump campaign handbook, skipping debates and focusing his attention on Senator Jacky Rosen, the Democratic incumbent, rather than on his Republican rivals.

That changed on Thursday, when Mr. Brown appeared to address Mr. Gunter for the first time, at a private fund-raising event in Sparks, Nev., after Mr. Gunter accused him in ads and appearances of being disingenuous and not sufficiently pro-Trump.

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