Republican Tim Sheehy bolstered the GOP's newly acquired Senate majority with a victory over three-term Democratic U.S. Sen. Jon Tester in a contest of national importance that featured a record-setting torrent of spending by the two sides.

Tim Sheehy, a wealthy Republican businessman, was dogged by controversies during the campaign. But Senator Jon Tester, a Democrat, could not outrun the demographic shift in his deep-red state.

Republican Tim Sheehy has defeated Democratic Sen. Jon Tester in Montana, NBC News projects, deepening the GOP’s projected Senate majority. Sheehy, a former Navy SEAL, defeated the three-term senator by consolidating Republican voters in a state that has ...