DULUTH, Minn.–Kansas independent Greg Orman isn’t the only third-party candidate making waves in this year’s midterm elections.

In Minnesota, a Green Party challenger could potentially tilt the race away from Democratic incumbent Rep. Rick Nolan, according to local experts and operatives on both sides of the aisle.

Ray “Skip” Sandman’s anti-mining stance could siphon votes from environmentalist Democrats who would otherwise back Mr. Nolan. The Democratic incumbent is currently locked in a tight race against Republican businessman Stewart Mills.

“His chances of becoming a congressman are nil and all he’s doing is improving the chances of the Republican,” Don Bye, chairman of the district’s Democratic-Farmer-Labor party said of Mr. Sandman. If the race comes down to the wire, Mr. Nolan would need to win by a wider margin if he loses some votes to Mr. Sandman, Mr. Bye said.

Mr. Sandman has staked out firm opposition to a copper-nickel and precious metals mine proposed by PolyMet Mining Corp.POM.T +1.71% currently under environmental review by the state. PolyMet has estimated the mine would create 360 direct jobs.

“The environment is a very special thing here and very delicate and at this point we do not have the science to clean up the spills,” Mr. Sandman, a Vietnam veteran and former corrections officer, said in a recent interview during his lunch break from his full-time job as a cultural adviser at a substance abuse treatment center in Sawyer, Minn.

That stance sets Mr. Sandman apart from Mr. Mills, who supports the PolyMet project, and Mr. Nolan, who said he believes technology has advanced enough to enable mining to proceed without harming the environment, so long as appropriate regulations are followed.

“I am supportive of mining, as long as it’s done right in protecting our air, our water, our health, our safety,” Mr. Nolan said.

That stance will play well in the district’s Iron Range, but may fall short with some voters in Duluth, the left-leaning urban center whose turnout will be crucial in deciding the winner, said Aaron Brown, a Northern Minnesota political blogger.

Mr. Sandman “has no chance [of winning] and isn’t that organized, but he’s pretty well regarded in the environmental community. He’s a viable protest valve that they can use,” Mr. Brown said.

The race between Messrs. Nolan and Mills is expected to be close. This week House Democrats pulled money from some races where hopes of victory are dimming, redirecting the funds to districts still in play, including in Minnesota.

Mr. Mills said the threat Mr. Sandman poses to Democrats illustrates the conflicting agenda from the party’s local labor and environmental wings.

“The Democratic party has a major problem: a blue-green divide,” he said. “Republicans have been unified in our message that we are for growing our economy in an environmentally sustainable fashion.”

Mr. Sandman said he doesn’t consider himself a spoiler in the race.

“What I’m offering the voters is a choice: do you wish to sacrifice your future of clean water for a few hundred temporary jobs?” he said.