Two Republican incumbents lost in the Montana Senate primaries on June 2. This was the first year since 2010 in which more than one incumbent lost in the chamber's primaries.

At the start of Montana's 2025 state legislative session, nine Republican senators voted with the chamber's Democrats to change procedural rules for the session. In May 2025, KTVH's Jonathon Ambarian wrote, "It was only the start of what became a session-long storyline, as that group of nine went on to vote against GOP leadership on numerous procedural motions and some key legislation." The Montana Republican Party censured the nine senators in April 2025.

The Daily Montanan's Micah Drew wrote after the March 2026 filing deadline for this year's legislative elections, "The well-documented rift in the Republican party between moderate members, including the infamous 'Nasty Nine' senators who worked with a bipartisan coalition in 2025, will be on display this spring as more conservative candidates seek to oust moderates."

Five of the nine Republican senators' districts are on the ballot in 2026. Of the five, one incumbent — Shelley Vance — ran for re-election, two — Jason Ellsworth and Bruce Gillespie — ran for state House, and two — Wendy McKamey and Russel Tempel — did not run for either office. That group of four incumbents that did not run for re-election in the Senate accounted for 40% of the chamber's 10 retiring Republicans. Vance lost re-election to Caleb Hinkle 74%-26%.

In an interview with the Bozeman Daily Chronicle after the 2025 legislative session, Vance said: "I’m a Republican in the way I see a Republican. I’m a Reagan Republican. A big-tent kind of Republican and our party needs to do that. If our party doesn’t and we decide we just have to stick to the straight line and walk that straight line, I don’t want to be part of the Republican Party. It would be a huge mistake for the Montana Republican Party to go in that direction, and I think it would dissolve itself."

Heading into the elections, Republicans had a 32-18 majority in the Senate and a 58-42 majority in the House.

Montana's term limits prevent legislators from serving more than eight years in a 16-year period in one chamber. Legislators can serve eight years in one chamber then eight years in the other without violating this rule. State representatives ran in eight of the nine battleground districts.