The Montana house is set to take up a bill that would change election rules in state — but only for the 2024 Senate election — in an effort to beat Senator Tester, one of that body’s most vulnerable Democrats, and snub the state’s Libertarians.

Montana SB 566 is going through the second committee in the state house, meaning it will soon be moving to the floor for a vote if approved.

The Montana senate passed the bill, 27 to 23, earlier in April, with seven Republicans opposing the bill alongside 16 Democrats. Republicans hold the state house, 68 seats to 32.

The bill’s sponsor, Senator Greg Hertz, has sold the legislation as a “test run” for a “jungle primary” system in the state, the same as many conservatives and Republicans panned in California.

“It always bothers me that sometimes our statewide elected officials don’t end up with the majority vote,” Mr. Hertz said about the bill.

The bill would remedy this by instituting a top two primary system, meaning a field of candidates would all run against each other in a single primary and the top two vote getters would advance regardless of their political party.

Ostensibly, the bill is meant to ensure that there are no spoiler candidates in elections that are won without a plurality of the vote. Critics say the bill is something else.

“Let’s just call this bill what it is — nothing but a partisan power grab,” a Democratic state senator, Ryan Lynch, said on the floor. “It’s not insignificant that there’s one statewide elected official who’ll be part of this.”

Per the text of the bill, the only election that would have its rules changed is the 2024 U.S. Senate election. The bill would sunset in 2025. There are no plans to extend it, even though the bill was sold as a test run for a new system.

A state senator, Carl Glimm, defended the bill on the floor, saying that “the only reason you should be scared of it is if you don’t think your guy can win.”

In effect, the bill would make the general election a head-to-head race between Montana Democrats and Republicans, cutting out the Libertarian Party, which has been significant in past statewide elections.

Mr. Tester has won with a plurality of the vote in the past, defeating Senator Burns 49 percent to 48 percent in 2006 and Lieutenant Governor Denny Rehberg 48.6 percent to 44.9 percent in 2012. A Libertarian candidate, Dan Cox, received 6.6 percent of the vote in 2012.

In 2018, Mr. Tester’s most recent re-election run, he won with a majority of the vote, defeating the state auditor, Matt Rosendale, 50.3 percent to 46.8 percent. Libertarian Rick Breckenridge received 2.9 percent.

This election, Republicans appear to be hoping that eliminating third party candidates will deliver them the Senate election, though that outcome is far from certain.

In the 2018 election, Republicans still would have lost to Mr. Tester if Mr. Rosendale received every single Libertarian vote. In 2012, transferring all of the Libertarian votes to the Republican candidate would have given the GOP the win, though whether this strategy proves fruitful in 2024 depends greatly on who is voting for those Libertarian candidates.

A data scientist at Decision Desk HQ who is a professor of data science at Washington University in St. Louis, Liberty Vittert, told the Sun that Libertarian voters tend to go to the polls regardless of whether a Libertarian candidate is in the race, but independents supporting a Libertarian candidate might not.

“Libertarians lean by almost two-fold toward Republicans so this is most likely a great thing for any Republican running,” Ms. Vittert told the Sun.

If the bill passes the Montana House and is signed by Governor Gianforte, Republicans will have gained another advantage in the Montana Senate race, which is likely to be one of the most contested seats in 2024.

Alongside Senators Manchin and Brown, Mr. Tester will be one of the most vulnerable senators in the 2024 election on the Democratic side, meaning that the few points Republicans win by cutting out third parties could make the difference.

Still, Mr. Tester has projected confidence that he can pull off a win for Democrats in deep red Montana once again, after having done so three times already.

“I know that people in Washington don’t understand what a hard day’s work looks like or the challenges working families are facing in Montana,” Mr. Tester said in his re-election announcement. “Montanans need a fighter holding Washington accountable.”