Republicans rejected the proposal in a special legislative session.
A last-ditch effort by Colorado's governor to give gay couples in the state rights similar to married couples failed Monday after Republicans rejected the proposal during a special legislative session.
Democratic Gov. John Hickenlooper had said the special session was needed to address a "fundamental question of fairness and civil rights."
The bill's demise was expected by Democrats, who have begun using the issue as a rallying cry to topple Republicans in the November elections. Republicans assigned the bill to House State, Veterans and Military Affairs Committee, which was likely to reject it. The panel voted 5-4 along party lines to kill the measure.
"The gay community is being used as a political pawn," said Republican Rep. Don Coram, whose son is gay. Coram voted against the measure.
Rep. Mark Ferrandino, the Democrats' leader in the House and gay lawmaker co-sponsoring civil unions, sounded a note of optimism before the committee hearing, even as he braced for the bill's rejection.
"If it fails this year, we're going to work hard to make sure the public understands what happened, the games that were played, and next we're going to push it again," he said. "And as I've said a number of times, it's not a matter of 'if,' it's a matter of 'when.' And the 'when' keeps getting sooner and sooner. This will happen."
House Republicans hold a 33-32 voting advantage, but there was enough support for civil unions to pass.
Last week, Democrats tried to force Republicans who control the calendar to bring up the bill for debate. But it became clear Republicans were filibustering by unnecessarily talking at length about other bills.
Republicans then halted work for hours, killing the bill and several others that needed a vote before a key deadline.
"Transparency, accountability and the virtues of good government are compromised when the legislative clock is used to avoid consideration of important legislation," the governor said in a letter to lawmakers before the special session started.
The regular session adjourned Wednesday, the same day an emotional Hickenlooper announced his intent to call a special session over civil unions and other bills that died because of the GOP filibuster.
More than a dozen states allow either gay marriage or civil unions, including several that moved to pass such laws this year.
The debate in Colorado is playing out at a time when President Barack Obama became the first U.S. president to publicly endorse gay marriage. But North Carolina voters approved a constitutional amendment that bars civil unions and defines marriage as solely between a man and a woman.
Earlier Monday, hundreds of supporters wearing red and waving signs greeted lawmakers returning to Denver for the special session.
Many of the gay-rights activists predicted the bill's demise. They urged Democrats to make the civil unions failure a rallying point for November.
"For too long Democrats have let the right corner the market no talking about values. Finally we've claimed the moral high ground, and we can talk about that," said activist Wiley Sherer, who was selling buttons that read, "Ignorance is forgivable. Pride in ignorance never is."
Undeterred by the civil unions delay, Sherer said, "It's going to happen eventually. I mean, there's no way in 10 years we don't have nationwide marriage equality."
Conservatives, who argue civil unions undermine traditional marriage, portrayed the special session as a waste of taxpayer money. As they have before, Republicans described the measure as a proposal for gay marriage, rather than civil unions.
Civil unions would grant gay couples rights similar to married couples, including letting partners make medical decisions for each other. The protections also would enhance parental and inheritance rights.