LAS VEGAS—Donald Trump took his economic nationalism and politics of personal confrontation to Nevada on Saturday, testing whether his campaign style can help Republicans in a state carried by his Democratic rival Hillary Clinton in 2016.

Mr. Trump made the trek to Las Vegas to headline a fundraiser for Sen. Dean Heller, the only Republican senator in the state seeking re-election this year.

“He was with me all the way—once we got elected,” Mr. Trump said, repeatedly recalling Mr. Heller’s delay in backing his bid for the White House. “A little bit shaky in the beginning.”

Mr. Heller “cut your taxes and nobody fought harder to cut your taxes than Dean Heller, let me tell you,” Mr. Trump said. The Democrats, he said, “want tax increases. They want open borders.”

Mr. Heller’s race is one of the most consequential Senate contests of the year, as Republicans seek to hold on to their 51-49 majority in November’s elections. Nevada, a swing state, will be critical come November, with a Senate seat, the governor’s office and two competitive House races on the ballot.