Tester, a three-term incumbent, faced little opposition in the Democratic primary, winning his race with more than 97% support.
Sheehy, a former Navy SEAL and businessman who had been endorsed by former President Donald Trump, also cruised to victory, garnering 73.5% support in the GOP primary.
“It’s official. I’m facing off against Mitch McConnell’s handpicked candidate Tim Sheehy for Montana’s U.S. Senate seat,” Tester wrote on X Tuesday night. “And I’m going to win.”
“I have farm equipment that’s been in Montana longer than Tim Sheehy,” the 67-year-old senator snarked in a separate post.
Despite his confidence, the Montana Democrat faces a tough re-election battle in a deeply red state.
Tester is the only Democratic member of Montana’s congressional delegation.
The state also has a Republican governor and both chambers of the statehouse are GOP-controlled.
Montana voters backed Trump in the 2020 presidential election by a 16.4 percentage-point margin, and by a 20.4-point margin four years earlier.
Tester’s previous general election victories have also been narrow.
In 2006, he was elected to the Senate by a 0.9 percentage point margin. He did better in 2012, winning re-election by 3.7 percentage points, before falling back a bit in 2018, when he emerged victorious by only 3.5 percentage points.
“America is at a crossroads and we need a new generation of leaders to save our country,” Sheehy wrote on X after his primary win.
“I am humbled and honored by all the support and look forward to finally retiring the #1 recipient of lobbyist cash and pro-Biden liberal #RetireTester” the 37-year-old candidate added.
Tester has a 5.5-point edge over Sheehy, according to a RealClearPolitics average of polls, though polling in the race has been limited to only a couple of surveys, and none since March.
“I am humbled and honored by all the support and look forward to finally retiring the #1 recipient of lobbyist cash and pro-Biden liberal #RetireTester” the 37-year-old candidate added.
Democrats currently have a 51-49 edge over Republicans in the upper chamber.