Montana’s Republican party leadership is opposing a Republican-sponsored measure to reform the state’s elections, warning that it would “give Democrats an inherent advantage” due to their ability to increase voter turnout door-to-door.

In an email titled Emergency Chairman’s Report, the Republican party chairman, Jeff Essmann, set off a furious war of words, with Democrats accusing Republicans of attempting to suppress the vote because it might mean a loss for the party.

The dispute focuses on a bipartisan bill before the Montana legislature that would make an upcoming election to replace Representative Ryan Zinke, a Republican nominated by Donald Trump to be interior secretary, an all-mail ballot vote.

Essmann warned that if the bill passed, the Democrats would have an advantage “in close elections due to their ability to organize large numbers of unpaid college students and members of public employee unions to gather ballots by going door to door”.

“This a Republican saying, no, let’s not let everybody vote,” said Nancy Keenan, the state’s Democratic party leader. “This is wrong, and it is wrong that he would attempt to suppress votes.”

The measure’s sponsor, Republican senator Steve Fitzpatrick, has said concerns that Democrats would unduly benefit are overstated. “It’s important that we have as much of a chance as we can to get people out to vote as well,” Fitzpatrick said.

The election, Fitzpatrick added, comes at an unusual time. The all-mail ballot, which only covers the 2017 special congressional election, was merely designed to save counties as much as $500,000. The mail ballot has been in place in Montana for a decade, but this proposal would eliminate in-person voting for this special election.

“It is a little amusing – Republicans are always saying they’re fiscal conservatives but they want a system that’s costing counties a lot of money,” Keenan said.

But Essmann, who is also a state house representative from Billings, denied that the warnings to Republican party members amounted to an attempt at voter suppression.

“Senate Bill 305 removes an option from voters and that’s detrimental to many Republican voters, but also many Native American voters many of whom support Democrats who prefer to vote at the polls. I think every voters should have every option to vote.”

In his email, Essmann also said “vote-by-mail is designed to increase participation rates of lower propensity voters. Democrats in Montana perform better than Republican candidates among lower propensity voters and Republican candidates do better among higher propensity voters.”

That prompted an immediate response from Keenan. She called the letter “deeply troubling”, saying it showed “their party’s desire to suppress the voices of Montanans in this special election”.

“The fact is there is no definitive data on mail ballots but the effort here is to suppress people from voting and having their voices heard in an election,” Keenan told the Guardian.

Essmann described Keenan’s claims as “preposterous”.

“It’s an outrageous charge and totally false,” Essmann said. He also suggested that Democrats, not Republicans, had engaged in unethical practices, including going door to door collecting ballots.

Essman held a meeting on Wednesday warning that Democrats had “perfected the mechanics of using mail ballots”, and he feared “the long-term viability of our Republican Party” if the state switched permanently to an all-mail voting system, according to the Bozeman Daily Chronicle.

Later that day, the bill passed a senate committee. It will now go to the full state senate.

Disagreement within the Montana Republican leadership extends to the secretary of state, Corey Stapleton. According to the Bozeman Daily Chronicle, Stapleton advised senators to think twice about “an all-mail-in ballot”.

“If you look at the three states that have done it, you can see that populism and direct democracy at its best, all three states – Oregon, Washington and Colorado – they do all-mail-in ballots and they’re all marijuana-all-the-time states too. Is that what you want? Because that’s what you’re going to get.”

Opposition to SB 305 has also intensified from Democratic quarters in recent days. Representative Sharon Stewart-Peregoy told the Billings Gazette that all-mail elections would harm Native Americans. “I am highly opposed to this as the suppression of the Indian vote in my district and I believe it’s another example of a government speaking with a forked tongue,” Stewart-Peregoy said.

“The issue at hand is to allow people to vote and we should do everything possible to engage in voting,” Keenan said. “Essmann has made it a partisan issue; he’s saying, ‘We just want some people to vote.’ That’s what I find offensive, that’s what I think is wrong, and what I think most Montanans find very discouraging.”