In keeping with Colorado’s independent streak, about 21 percent said they were not planning to vote for either candidate or were undecided — a group to which Mr. Coffman appealed when he also refused to support Mrs. Clinton.

Mr. Coffman will have to do more than just retain the favor he has gained in his increasingly Democratic district, Mr. Wasserman said, because more Democrats turn out to vote in presidential election years.

And this year he is grappling with another factor: “a very credible and talented opponent,” Mr. Wasserman said.

That opponent, Morgan Carroll, a Democrat who served as president of the State Senate, listened intently on Aug. 6 as a couple of dozen Latino activists and voters enumerated their concerns at her campaign office as they munched on pan dulce, a Latin American pastry, and doughnuts. Grievances included a fractured education system, a flawed criminal justice system and a broken immigration system. She jotted down notes with a purple gel pen, letting her guests steer the conversation but slipping in to offer her position or a brief interpretation of the congressional landscape.

Ms. Carroll, who grew up in Colorado and has represented parts of the district in the state legislature since 2005, is a self-described policy nerd, dropping to a whisper as she confesses that. Ms. Carroll has read every bill she has ever voted on, she said, and a few she has not, such as the entire Affordable Care Act, she said — twice.

In the legislature, Ms. Carroll sponsored legislation requiring universal background checks for firearms sales after the 2012 movie theater shooting in Aurora that left 12 dead and scores injured. Her bill became law in 2013.