LAS VEGAS—No Republicans on the ballot in 2018 have shifted their Trump-era political allegiances more than Nevada Sen. Dean Heller.

Mr. Heller never endorsed President Trump’s 2016 campaign and declared himself “99% against Trump.” During Mr. Trump’s first months in office, Mr. Heller called a press conference to announce his opposition to the Republican health-care overhaul Mr. Trump promised.

Then...