Sen. Jon Tester’s (D-Mont.) reelection was already difficult. It just got a little harder.

The Montana state Supreme Court upheld a lower court ruling Tuesday that Green Party candidate Robert Barb can remain on the ballot for this fall’s Senate race, a blow to state Democrats who had fervently tried to block him out of fear he would pull votes away from the highly vulnerable Tester.

Of the incumbents running for reelection, Tester is locked in the most difficult race in a state that Donald Trump won handily in 2020. He faces Republican Tim Sheehy and two third-party candidates, Barb and a Libertarian.

Barb replaced the original Green Party nominee after the primary winner dropped out of the race. The state Democratic Party argued that proper procedure was not followed, and that Barb should be booted from the ballot. Their efforts clearly underscored the threat they believe Barb could pose to Tester.

There’s some evidence that’s the case. A recent AARP-commissioned poll found that Sheehy led Tester by 6 points in a head-to-head ballot but by 8 points when the Green and Libertarian nominees were included. (Though they tested Michael Downey, the initial Green Party nominee, in that poll.)

In their lawsuit, the state Democratic Party said that a candidate swap would force them “to divert staff time and resources to developing new messaging strategies that appeal to voters choosing between the Democratic and Green Party candidates.”

There was no Green Party candidate on the ballot in Tester’s 2018 race, when he won by 3.5 points. But there was a Libertarian candidate on the ballot who drew 2.9 percent of the vote.