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BILLINGS, Mont. (AP) — Libertarians lined up with Democrats on Friday against a proposal that would effectively block third party candidates from next year’s Montana U.S. Senate election, as Republicans try to consolidate opposition to incumbent Jon Tester in a race pivotal for control of the Senate.

Republicans who control the state Legislature want to alter the 2024 Senate primary in Montana so that only the top two candidates, no matter their party, advance to the November election. Past races for Tester’s seat were close enough that many Republicans blamed third party candidates for draining away potential GOP votes and giving the Democrat the victory.

Critics blasted the proposal during a Montana House committee meeting as a blatant attempt to rig the election.

“It will completely block us out,” said Montana Libertarian Party Chairman Sid Daoud. “It’s going to eliminate any third party or independent candidate from moving to the general election.”

Democrats hold a narrow 51-49 U.S. Senate majority. They will be defending 23 seats next year including in some states — Montana, West Virginia and Ohio – that have grown increasingly Republican. The GOP will be defending 10 seats and also trying to flip seats in swing states such as Nevada, Pennsylvania and Wisconsin.

Republicans are eager to deny Tester a fourth term after GOP candidates dominated recent elections in Montana. Tester is the only Democrat now holding statewide political office.

The Republican sponsor of the proposed primary changes, state Sen. Greg Hertz, said during Friday’s hearing that he wanted to stop the GOP and Democrats alike from trying to manipulate elections by stealthily promoting third party candidates.

Emails included in the legislative file for the bill show that longtime Republican lobbyist Chuck Denowh helped shape the measure.

On March 26, Denowh suggested to Hertz, a second lawmaker and legislative staff that an early version of the measure be changed so that it would apply only to the U.S. Senate and would sunset after the 2024 election, the emails show. The bill was changed accordingly.

However, lawmakers are now considering an amendment to the bill that would make it apply to future Senate races. Primary races for other seats would be unchanged in 2024, with voters in each qualifying political party continuing to select their own candidates for the general election.

The Montana House State Administration Committee planned to vote on the measure, and the amendment, on Monday.

The state Senate approved the 2024 primary change by a 27-23 vote last week. Seven Republicans joined all of the chamber’s Democrats in voting against the measure.

Both major parties in Montana have sought to use third parties to their advantage in past elections.

During one of last year’s U.S. House elections, a Democratic-linked group in Washington, D.C., sent mailers promoting Libertarian candidate John Lamb as the race’s “true conservative” in an effort to peel away conservative votes.

Liberal-backed groups used similar tactics during Montana’s 2012 and 2018 U.S. Senate races.

In the 2020 race, the Republican Party bankrolled a $100,000 signature-gathering effort to put the Montana Green Party on the ballot. The state Supreme Court removed the Green Party after hundreds of people sought to withdraw their signatures upon learning the GOP was behind the effort.

No one has claimed responsibility for a 2018 signature-gathering effort to get the Green Party on the ballot. That effort was overturned when a judge invalidated enough signatures so the party didn’t qualify.

“Some major parties probably want to continue doing this, including my own,” Hertz said Friday. “Let’s get away from this, what the two major parties are doing in Montana.”

Tester is considered highly vulnerable in 2024. Montana voted decisively for President Donald Trump in 2020 and Republicans last year claimed the state’s newly created U.S. House seat, with a win by former Trump Interior Secretary Ryan Zinke.

Zinke and fellow Republican Rep. Matt Rosendale are considered potential challengers to Tester next year.

But the Democrat has proven a formidable campaigner and prodigious fundraiser. His campaign this week reported raising $5 million in the latest quarter.