Within hours, newspapers in Missoula, Billings and Helena had rescinded their endorsements of Mr. Gianforte, House Democrats released a digital ad featuring the audio recording, and Republicans were in a state of paralysis about what to do with a candidate who suddenly had a court date next month.
At a final rally for Mr. Quist in a brewpub in Missoula, activists were electrified by the news, and some of them said they intended to play the tape for those yet to vote when canvassing for Mr. Quist.
Washington-based Republicans were already grumbling about having to spend millions of dollars on behalf of Mr. Gianforte, who is a multimillionaire. Montana still occasionally elects Democrats statewide, but it leans Republican and has not sent a Democrat to the House for over two decades.
Republican groups have sought to support their nominee by attempting to focus the race largely on Mr. Quist’s personal financial difficulties, lashing him in a well-financed advertising campaign. Yet Mr. Gianforte has made it difficult for his party, which is concerned about the intensity of liberal voters in Montana and beyond who are hungry to send a message to President Trump.
Mr. Gianforte irritated Republican officials earlier this month when he said on a conference call with Washington lobbyists that he was “thankful” that the House had passed legislation to repeal the Affordable Care Act, after suggesting to Montana voters that he would not have supported the bill. Democrats have seized on an audio recording of that call, pointing to his comments as evidence that he is sending contradictory messages to different audiences.
Now he is facing another audio recording that could prove even more threatening. The question, though, is whether there are enough outstanding ballots to make a difference in the race.
Voting by mail is common in the state, and officials in both parties believe that more than half of the total ballots that will be cast in the election had been submitted before Thursday.
The race had become nationalized well before the fracas on Wednesday.
Mr. Gianforte unreservedly embraced Mr. Trump, highlighting his efforts to build relationships with the new administration in a wager that the president’s tumultuous first months will matter little in a state that he carried by 21 points last year. Mr. Gianforte, a wealthy technology executive, has campaigned with Vice President Mike Pence, welcomed Donald Trump Jr. to the state for two multicity tours and had the president and vice president record automated calls for him in the campaign’s final days.
“I’m running to go back there to be a voice for Montana and to work with Donald Trump,” Mr. Gianforte said in an interview Wednesday. “My opponent clearly has said he’s going to obstruct Donald Trump. That’s the decision here.”
If Mr. Gianforte is trying to test the depth of Mr. Trump’s appeal in a heavily rural state with an enduring populist streak, Mr. Quist is seeking to demonstrate just how much of a political liability the House-passed health care bill is for congressional Republicans. Mr. Quist, a banjo-strumming folk singer, has refocused his campaign in its final weeks on the Republican efforts to repeal the Affordable Care Act.
The contest in Montana, to fill the seat vacated by Interior Secretary Ryan Zinke, has drawn national attention, with both sides together pouring over $10 million into television and radio ads. But this spending in Montana’s relatively cheap media markets happened almost in spite of the national Democratic Party, which has been skeptical about Mr. Quist’s prospects. Democrats only began helping their nominee here reluctantly, after weeks in which Republicans hammered Mr. Quist on TV with little response. Republicans outspent Democrats more than two-to-one on television and radio, according to media buyers in both parties.
This disparity and the Republican tilt of Montana make the race less of a bellwether than the special House election next month to fill the seat formerly held by Tom Price, the secretary of health and human services, in suburban Atlanta. But after Democrats’ closer-than-expected loss in March in a special House election in Kansas — in a district that is even redder than the one in Montana — the contest here will be closely watched for insights about just how perilous a political environment Republicans face going into 2018.
Where to Watch
An old rule in Montana politics — that the winner of a statewide election could be gleaned by watching who won Yellowstone County, home to Billings — has receded in recent years. But the county, Montana’s most populous, still merits a close eye. If Mr. Gianforte does not amass a significant margin of victory there, the race could prove competitive. Similarly, if Mr. Quist does not come out of Cascade County, where Great Falls sits, with a substantial advantage, it will be difficult for him to keep the contest close.
But the most important task for Mr. Quist in a special election where turnout is so crucial may be to drive up his margins in the two liberal-leaning counties that are home to Montana’s large state universities: Missoula and Gallatin.
A Libertarian in the Mix
As Montana Republicans know all too well, Libertarians here have a history of siphoning enough votes to hand statewide races to Democrats. And there is a Libertarian, Mark Wicks, on the ballot Thursday. While Democrats have not overtly tried to promote Mr. Wicks, as they have Libertarian candidates in previous elections, both parties acknowledge that he could capture about 5 percent of the vote — enough to make a difference.
Prepare for a Long Night
If we have piqued your curiosity about this race, here is a suggestion: Get some coffee. It’s going to be a long night. The polls close at 10 p.m. Eastern, and The Associated Press is often slow to make projections in Montana.
The race for governor in November was not called until 10 a.m. the next day, even though Steve Bullock, the Democrat, was re-elected by a fairly comfortable four-point margin. Similarly, the Democratic presidential primary wasn’t called until after 2 a.m.; Bernie Sanders ultimately won by seven points.
Why? It often takes a long time for Montana’s conservative and rural areas to report. It is also tricky because so many ballots are cast by mail. More than 250,000 have been returned already, a significant enough portion of the just over 699,000 registered voters in the state to potentially mitigate the effects of Mr. Gianforte’s assault charge. The substantial early vote will also make it harder to get a quick grasp on the race, since the mailed votes can differ from those cast on Election Day.
What It Will Mean
With Mr. Gianforte centering his campaign on health care, and a new report from the Congressional Budget Office projecting that 23 million Americans would lose coverage if the House bill became law, a Democratic victory or even a narrow loss here could prompt congressional Republicans to recalibrate their approach to the issue. Few Republican leaders want to hand Democrats a made-to-order issue to campaign on across the country, no matter how conservative the state or district.
Beyond policy, a defeat or close call here for Mr. Gianforte would underscore the intensity on the left, bolstering Democrats’ effort to recruit candidates to run next year and nudging some Republican incumbents toward retirement. And if Mr. Quist does fall short by a small margin, it will inflame tensions between energized progressives determined to leave few districts uncontested and more pragmatic party officials loath to spend money in forbidding territory.
But Don’t Overread It
This year’s special elections have been heralded as a critical test of whether Democrats can retake the House next November. But it can be a mistake to read too much into the results of a single contest, and whatever happens on Thursday, it is clear that the national political environment is favorable to Democrats.
Republicans have already struggled on conservative terrain this year: winning the House seat in Kansas by just seven points after carrying it by 32 points last year, and watching Jon Ossoff, a Democrat, capture over 48 percent and nearly avert a runoff in the first round of voting last month for Mr. Price’s seat in Georgia. The Democrats benefited from strong turnout and grass-roots fund-raising in both contests.
Given Mr. Trump’s large margin of victory here, even a fairly comfortable win for Mr. Gianforte would still be consistent with the emerging pattern of Democratic strength.
The strong performances by Democrats are not especially surprising with the president’s approval rating in the high 30s. They suggest that the House may be in play next November if Mr. Trump’s political fortunes don’t improve.
The Democratic Party’s ascendant populist wing, led by Mr. Sanders, believes that a more ambitious economic message can lure back Trump voters. But in 2018, there’s a catch: Most of the competitive House districts are in well-educated and affluent suburbs where Mr. Trump fared poorly last November.
Mr. Ossoff’s success in the Georgia race, in one of the most well-educated districts in the country, has already shown that Democrats have a path forward in traditionally Republican parts of the Sun Belt. A solid performance by Mr. Quist in Montana, an overwhelmingly white and working-class state, could do the same for populists who argue that the party can win back such voters with the right kind of candidate.
Mr. Sanders campaigned for Mr. Quist last weekend across the state, drawing thousands, and his supporters here and beyond will surely claim some of the credit for a strong showing.