The special election for Montana’s sole House seat is becoming a test between two rival brands of populism: President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE’s and the progressive version embodied by Sen. Bernie Sanders Bernie SandersSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Schumer, Sanders call for Senate panel to address election security MORE (I-Vt.).

Vice President Pence will bring the Trump brand to Big Sky Country this week to fight for Republican Greg Gianforte, appearing after repeated visits from Donald Trump Jr.

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And Sanders is slated to campaign this month for Democrat Rob Quist.

The House campaign pits each party’s ascendant populist strain against the other’s. Trump’s economic nationalism took him to the White House. Sanders’s left-wing populism helped him compete with Hillary Clinton Hillary Diane Rodham ClintonBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Virginia Democrat blasts Trump's 'appalling' remark about COVID-19 deaths in 'blue states' The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE in the Democratic presidential primaries and then launched him into a pivotal role in the defeated party’s rebuilding effort.

Gianforte, a millionaire tech entrepreneur, is the favorite in the race. But polls show Quist, a local folk musician, gaining ground. The May 25 special election will fill the seat vacated by Interior Secretary Ryan Zinke.

Both candidates’ strategies borrow from their respective party standard-bearers, a fact highlighted by the upcoming visits from national figures.

“Libertarian and individualistic impulses are much stronger in the West in states like Montana that don’t have a lot of population density,” said Jeremy Johnson, a political science professor at Carroll College in Helena. “The rugged individualism plays here.

“You’ve got this proxy fight. You don’t want to be a Washington insider; you want to be appealing to the average voter who has little to do with the inside-the-Beltway politics. What you’d call ‘right-wing populism’ and ‘left-wing populism,’ they both speak to different voters but with a similar impulse.”

Pence’s Friday swing through Billings, Mont., brings the vice president to a state his ticket won by 20 points in 2016.

Gianforte has played up his ties to the White House throughout his campaign, saying he’d be another Trump ally in the House.

Donald Trump Jr. returns to Montana on Thursday with a jam-packed day of rallies across the state. The avid hunter campaigned with Gianforte last month, urging voters to elevate Republicans in the state.

2016 provided mixed results for the GOP. While Trump won big, Gianforte lost his bid for governor to Democrat Steve Bullock. He can tie himself to Trump, but he’ll still have to convince voters who did not back him in November to change their minds — or at least drive up Republican turnout.

“The reason he lost his governor’s bid last year was that there were all these people who voted for Trump and Zinke who didn’t vote for Gianforte,” said Robert Saldin, a political science professor at University of Montana.

“Bringing out the vice president and Donald Trump Jr. … If that’s the group you are trying to appeal to, what better way to declare yourself a Trump Republican?”

Both Trump and Sanders promote staunchly anti-establishment principles. They oppose free-trade deals backed by both Democrats and Republicans, and they embrace the notion that party elites are losing touch with the rest of the country.

Those similarities have led to crossover appeal despite their major differences on issues such as healthcare and taxes.

A recent poll of Trump voters by the University of Virginia found that while Clinton’s favorable rating among them is just 3 percent, Sanders polls drastically better, with 21 percent viewing him positively.

“It seems totally bizarre to a lot of people who live and breathe this stuff … but there’s far more crossover among those groups than you might think,” Saldin said.

Quist hopes the Sanders appearance will work on two levels, both making inroads with independents or Republicans who may be open to crossing the aisle for Quist while also rallying the Democratic base.

Like Sanders, who isn’t officially a Democrat, Quist has tried to keep Washington Democrats at arm’s length. He’s staked out positions to the left of the national party, supporting a single-payer healthcare system and calling for trade agreements to prioritize American workers.

Sanders’s message resonated with Montana voters. He won the state by more than six points in the Democratic primary there.

Sanders’s frustrations with the Democratic establishment regularly bubbled up during that campaign. Similarly, Quist reportedly declined an appearance from Democratic National Committee (DNC) Chairman Tom Perez, according to the Huffington Post. Perez was seen as the establishment pick during the recent race for DNC chairman.

While Sanders’s appearance hasn’t been finalized, Montana Democrats said the event is still on. Scheduling has been complicated by Sanders’s schedule in the Senate, which won’t break for recess before the May 25 vote.

In the meantime, Sanders’s political group, Our Revolution, has already made a big push for Quist on social media and by making phone calls.

Turnout will be crucial for both parties because Election Day falls on the Thursday before Memorial Day, when Montanans are likely to be more interested in enjoying the long weekend than voting.

Many voters will go to the ballot box on Election Day, but Montana automatically sent absentee ballots to about half of all registered voters. By Monday evening, 88,000 absentee ballots — a quarter of those sent out — had been returned.

“Bernie’s message is very popular with a lot of people and resonates with a lot of people, and Rob also has a populist message,” a Montana Democrat familiar with the racesaid.

“There are a lot of similarities there in their message. It’s a get-out-the base election, so that’s what it’s going to come down to.”

The race is drawing spending to match the uptick in star power. Outside groups and campaign committees have spent more than $4.3 million so far, according to The Hill’s analysis.

Republicans have vastly outspent Democrats so far. In total, about $3.6 million has been spent on Gianforte’s behalf as of Tuesday, the majority in attack ads on Quist by groups like the Congressional Leadership Fund and the National Republican Congressional Campaign.

Quist’s allies have spent $680,000, about half of that from the Democratic Congressional Campaign Committee, which announced this week that it would increase its own investment to $600,000.

GOP attack ads look to play up Quist’s history of financial woes and his stance on gun rights. Quist previously proposed registering assault weapons but has since said he would protect Second Amendment rights — even appearing in a campaign ad shooting a TV with a rifle.

While Democratic spending is far below that of Republicans, most of the left’s spending has come over the past few weeks, tentatively soothing Democrats who had been frustrated with a lack of investment from the national party.

Democrats have seized on Gianforte’s public and private comments about the controversial GOP healthcare reform bill while playing up the fact that Gianforte grew up outside Montana, in New Jersey. But he’s lived in Montana for the past two decades.

The increased attention comes as surveys predict a tightening race. Early polls had Gianforte with a double-digit lead. But recent ones show Quist within single digits of his GOP opponent.

A Democratic poll conducted by Senate Majority PAC found Quist trailing Gianforte by only 6 points.

Polls from Gravis Marketing also suggest Gianforte’s lead is shrinking. A survey from late April found Gianforte with a 13-point lead, but a poll conducted a week later found his lead reduced to 8 points.

“It definitely has been noticeably tightening over the past week or so,” said a Montana Republican.

“I’m hopeful that Mike Pence Michael (Mike) Richard PenceGOP short of votes on Trump's controversial Fed pick Pence seeks to boost Daines in critical Montana Senate race The Hill's Campaign Report: Trump's rally risk | Biden ramps up legal team | Biden hits Trump over climate policy MORE and Donald Trump Jr. coming back and things like that coming to town will spike the base a little bit and get us back up kind of where we think we should be.”

Megan R. Wilson contributed.