Republican multimillionaire Greg Gianforte has won Montana’s only US House seat despite being charged a day earlier with assault after witnesses said he grabbed a Guardian reporter by the neck and threw him to the ground.

Gianforte, a technology entrepreneur, defeated Democrat Rob Quist to continue a two-decade Republican stronghold on the congressional seat.

Democrats had hoped Mr Quist, a musician and first-time candidate, could have capitalised on a wave of activism following President Donald Trump’s election.