Gianforte beat Democrat Rob Quist, a banjo player and first-time candidate who had focused his campaign on criticism of the Republican effort to repeal and replace former President Barack Obama's healthcare law. CNN projected Gianforte would win. With 96 percent of the vote counted, he led Quist by 51 percent to 43 percent.

Gianforte prevailed despite being charged on Wednesday night with misdemeanor assault on Ben Jacobs, a political correspondent for the U.S. edition of the Guardian newspaper, who said the candidate "body-slammed" him during a campaign event in Bozeman.

Gianforte's victory is a boost for Republicans, who are worried Trump's political stumbles and the unpopularity of the healthcare bill passed by the House will hurt their chances of holding on to a 24-seat House majority in next year's elections.