Montana's Democratic Gov. Steve Bullock, who is also running for US Senate this year, issued a directive last month allowing counties to expand voting by mail and early voting "at their local discretion," including mailing ballots to qualified voters.
Montana already allowed voters to request and submit absentee ballots without explaining their reasoning.
Bullock's approach to making ballots more accessible to voters during the pandemic "coupled with ... a nationwide push by the Democratic Party for the same measures, clearly reveals that the Election Directive is less about protecting the health of Montanans and more about enhancing the Governor's electoral prospects, along with those of his political party," the Republicans wrote in their suit filed Wednesday.
The Republicans argue that the procedures Montana has in place could allow for fraud and dilute legitimate votes.
The challenge isn't the party's first effort to limit mail-in voting, which President Donald Trump has frequently and baselessly assailed as prone to fraud. Last month, the Trump campaign sued New Jersey and Nevada for planning to send mail-in ballots to all state voters. And a US district court judge last week effectively shut down Trump's plan to sue Pennsylvania in federal court to determine how the state conducts mail-in voting.
Bullock cited such challenges in a statement on Wednesday, asserting that "This template lawsuit appears to be part of a pattern of lawsuits across the country by Republican party operatives to limit access to voting during the pandemic."
"Voting by mail in Montana is safe, secure, and was requested by a bipartisan coalition of Montana election officials seeking to reduce the risk of COVID-19 and keep Montanans safe and healthy," Bullock continued, adding that "Today, we have many more active cases of COVID-19 than we did back in June."
CNN has reached out to Republican Secretary of State Corey Stapleton, who is also named in the lawsuit.
Montana has also faced an intrastate legal battle over whether Green Party candidates for US House and Senate could appear on Montana ballots this fall. They likely will not, after the US Supreme Court declined last week to take up an emergency petition from Stapleton -- a decision that could have a major impact on the competitive Senate race between Bullock and Republican Sen. Steve Daines.
Montana Republicans earlier this year backed an effort to qualify the Green Party, which has traditionally drawn some liberal voters from the Democrats, for the ballot. But after that was revealed, state Democrats urged enough people who signed petitions to disavow their support.
The secretary of state's office, however, did not accept requests to remove their support, so the state Democrats and several people sued. A lower court judge and the state Supreme Court sided with the Democrats, saying the Green Party no longer qualified for November's contest.