For Democrats, though, the contest pointed to an increasingly heated disagreement over where the party has a realistic chance to win. Party officials in Montana and progressive activists beyond the state’s borders grew frustrated last month watching outside Republican groups savage Mr. Quist as Democratic groups remained on the sidelines.

After a special House election in Georgia in which the Democrat Jon Ossoff received more than 48 percent of the vote — nearly averting a runoff and demonstrating the extent of voter enthusiasm on the left — Senator Jon Tester, a Montana Democrat facing re-election next year, called Representative Ben Ray Luján of New Mexico, the chairman of the House Democratic campaign arm, and implored him to consider spending money on Mr. Quist in the final weeks of the Montana race, according to two Democratic strategists briefed on the call. Mr. Tester also contacted the Senate Democratic leader, Chuck Schumer of New York, to see if he would carry the same message to the House Democratic leader, Nancy Pelosi of California.

But House Democratic officials make no apology for their prudence, believing they are more likely to claim the 24 seats needed to capture the House majority in suburban districts with highly educated voters, where anger at Mr. Trump runs high. That includes districts like the one in suburban Atlanta, previously represented by Health Secretary Tom Price, where both parties have poured tens of millions of dollars into a contest that looms all the more consequential after the Democratic defeats in Kansas and Montana.

Even this week, just two days before the Montana vote, Mr. Luján announced new spending in the Georgia race. And in private, Mr. Luján was telling other House Democrats that Mr. Quist stood little chance, based on private polls showing Mr. Gianforte with a healthy, consistent lead of about 10 percentage points, according to one of those present at a closed-door meeting of the caucus. After the election was called, the Democratic Congressional Campaign Committee circulated a memo declaring that it had “refused to waste money on hype.”

On Friday, Democratic leaders emphasized that Mr. Quist had performed better than the party’s past congressional candidates in Montana, apparently benefiting from the enthusiasm of rank-and-file Democrats even as he fell well short of victory. The party’s nominees, they noted, are outpacing their predecessors on fairly forbidding terrain, and Democratic voters are participating at higher rates than Republicans, despite being outnumbered in these districts.

But other Democrats acknowledged that they must work harder to make inroads with voters who live far beyond major cities and their suburbs, if they want to pick up seats like the one Mr. Gianforte just captured.