He added, “I had a call yesterday from one of them just to ask, ‘If not now, when?’ ”

Brad Clark, executive director of One Colorado, a statewide gay and lesbian advocacy group, criticized Republican lawmakers. “Although a clear majority of Colorado voters support civil unions — and a clear majority of our representatives agree with them — House leadership killed the bill, proving just how out of touch they are with everyday Coloradans,” he said.

Members of the Republican legislative leadership, who are opposed to civil unions, argued that Democrats had forced a political showdown by waiting until the end of the session to push the legislation through.

At one point Tuesday night, Democratic lawmakers tried through a procedural motion to halt the debate on other bills after it seemed that Republicans were drawing out their comments.

“I’ll tell you why this bill moved at a different pace,” said Representative Mark Waller of Colorado Springs, a Republican. “Because the Democrats were playing politics with it.” Representative Mark Ferrandino of Denver, one of four openly gay state legislators here, then turned toward Mr. Waller, saying, “We’re willing to work with you to figure out a way to make sure that every bill, tonight, gets an up-or-down vote on it.”