Sen. Jon Tester of Montana is regarded as one of the Senate's most vulnerable Democrats, trying to defy the odds in a state that Donald Trump carried by 20 percentage points in the 2016 presidential election. (Associated Press/File)

Sen. Jon Tester[1] of Montana was already facing a tough re-election battle in a state that Donald Trump won by a landslide — and then the Green Party[2] made the Democrat’s job that much tougher by qualifying for the ballot.

Now the Montana Democratic Party[3] is scrambling to oust the Greens. It has asked a judge to decertify the left-wing party[4] by declaring 180 petition signatures invalid for reasons such as bad handwriting, use of initials instead of full names and failure to write in cursive.

The lawsuit, filed April, 2 has the Greens seeing red. They accuse state Democrats of trying to disenfranchise voters in a politically motivated attack.

“They’re scared,” said Danielle Breck[5], Montana Green Party coordinator. “There seems to be this belief that we’re going to spoil the race, particularly [for] Jon Tester[6]. His seat is at risk, and they’re afraid we’re going to siphon votes from them.”

Democrats have reason to be nervous. Mr. Tester[7], a two-term incumbent, is regarded as one of the Senate’s most vulnerable Democrats, trying to defy the odds in a state that Mr. Trump carried by 20 percentage points in the 2016 presidential election.

Four Republicans are running to challenge Mr. Tester[8] in Tuesday’s primary, but polls show the race has increasingly become a contest between state auditor Matt Rosendale and former Judge Russell Fagg, who stepped in after Interior Secretary Ryan Zinke took a pass.

A rangy farmer known for his crew cut, Mr. Tester[9] has won narrow victories by positioning himself as a centrist in order to pick up Republican votes. That strategy may be tougher to pull off in an election season when liberals are increasingly flouting Democratic Party[10] favorites to side with nonestablishment candidates.

The Green Party[11] has two Senate contenders on the primary ballot: Tim Adams, a former state Republican State Central Committee staffer, and Steve Kelly, the 2002 Democratic nominee for Montana’s House seat.

Mr. Kelly, who won the Green Party’s Senate endorsement at its convention, said “there’s a whole huge group of people” in Montana willing to deviate from the Democratic Party[12] script....

“The best way to describe it is that Bernie [Sanders] won Montana in 2016,” Mr. Kelly said. “So read it and weep if we’re on the ballot.”Montana Democrats are fighting to keep that from happening. In their lawsuit, filed shortly after Secretary of State Corey Stapleton approved the Green Party[13]’s petition in March, Democrats argued that 180 of the signatures submitted were invalid.To qualify for the ballot, a party must submit at least 5,000 valid signatures, including 55 to 150 from at least 34 of the state’s House legislative districts.The Greens submitted more than 10,000 signatures from 38 districts and county clerks

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