It was not enough to help Mr. Morse overcome the conservative outrage that erupted this winter as Colorado’s Democratic-controlled statehouse passed several gun laws over near-unanimous opposition from Republicans and Second Amendment advocates. Among other things, the new laws require background checks for private gun sales and limit ammunition magazines to 15 rounds.

Mr. Morse’s hand was on the tiller during much of that debate. A former police chief, he said he found himself in a position of not just rounding up votes, but actually explaining the mechanics of guns to fellow Democrats. He brought a magazine to show his colleagues how it worked. In an emotional speech in March, as the debate reached its peak, Mr. Morse stood on the Senate floor and spoke of gun violence and “cleansing a sickness from our souls.”

His opponents pounced on the remark, framing it as a sign of Mr. Morse’s disregard for his gun-owning constituents, as well as a symptom of the widening gap between Colorado’s urban Democrats and its rural Republicans.

Mr. Morse represented a slice of Colorado Springs that straddles those fault lines. His district is closely split among Democrats, Republicans and unaffiliated voters. And on Tuesday, despite huge voter-turnout drives and Obama-style neighborhood canvassing, more of Mr. Morse’s opponents showed up to cast him out.

The passions ignited by the vote were on full display on Tuesday, as opposing sides lined up side by side outside polling places here in Colorado’s second-largest city. They spoke of knowing survivors of the mass shooting inside the Century 16 theater in Aurora, Colo. Two of Mr. Morse’s sisters held up a banner, and complained that their brother’s opponents were twisting his record and his words in bitter attack ads.

A few feet away stood Steven Martin, 53, a recall supporter with a Beretta handgun holstered on his hip.