Because Republicans control the House, they hold a majority on the State, Veterans and Military Affairs Committee.
The committee killed the bill with a 5-4 vote just after 8 p.m.
While some Republican representatives do support civil unions, none on the committee do. All Republican members on the committee voted "no" on HB 1006 - known as SB 2 during the regular legislative session.
Rep. Don Coram (R-Montrose) referred to his constituents, who opposed same-sex marriage and took part in a statewide vote in 2006 that rejected domestic partnerships and amended the state constitution, effectively banning same-sex marriage.
Coram felt the civil unions bill would reverse that 2006 vote. As he voted against the bill, Corum also spoke about his gay son.
"He knows the situation and he knows that I love him very much. But he also knows that I'm a representative of all the people. And the majority of my district, they refer back to the 2006 vote and so that's why I voted for it," Coram said.
The bill would have legalized civil unions for two adults of any gender - allowing them to obtain many of the same rights granted to married couples under Colorado law.
House Minority Leader Mark Ferrandino (D-Denver) sponsored the legislation and spoke of disappointment but also resolve to continue the fight for civil unions.
"The will of the people and the role of the Democratic process has been thwarted. But we will continue to fight. As I've said many times, this is not a matter of if - it's a matter of when. And I will tell you that when keeps getting closer and closer and this will happen soon," Ferrandino said.
Colorado State Patrol troopers were stationed inside the Old Supreme Court Chambers at the Capitol to maintain order during the hearing, which was standing room only.
Testimony spanned from political heavy-hitters, to religious leaders, to ordinary citizens.
Former Denver Mayor Wellington Webb has a gay son who died two years ago. Webb, Denver's first African-America mayor, spoke in favor of civil unions and urged legislators to "be on the right side of history."
"This is the civil rights agenda for today," Webb said. "This is going to pass. The only question is who is going to have the courage to pass it."
Fran and Anna Simon, a Denver couple, held up a picture of their 4-year-old son Jeremy as they gave emotional testimony to the State Affairs Committee.
"I love this woman so much and all I want is the security of knowing that I'll be able to take care of her as she has taken care of Jeremy and me," Anna Simon said. "This is the person I believe God has chosen for me."
James-Daniel Flynn, chancellor of the Catholic Archdiocese of Denver, urged legislators to honor the wishes of Colorado voters, who passed a referendum banning same-sex marriage in 2006.
"This is not simply a religious issue, this is a matter of common sense," Flynn said. "An attempt to redefine marriage, as this civil unions bill does, should be vigorously challenged."
Brian Raum, an attorney for the Arizona-based Alliance Defense Fund, cited states like New Jersey, Connecticut, and California where groups that supported civil unions legislation later filed lawsuits to legalize same-sex marriage.
Raum also mentioned current pending lawsuits in Hawaii and Nevada.
"Achieving civil unions is a calculated step to achieving court-ordered same sex marriage," Raum said. "Opposing same sex marriage while supporting civil unions is akin to the Trojans dragging a wood horse into the middle of Troy."
Raum and other opponents argued passing civil unions would eventually unravel the sanctity of marriage.
Opponents even argued that passing civil unions would actually give the gay community something akin to second-class citizenship, by providing separate but unequal rights to same-sex couples.
Gov. John Hickenlooper (D-Colorado) supports the bill and would have signed it into law. He ordered the special session last week. He said a special session over civil unions and other bills killed after a filibuster from Colorado House Republicans would address "a fundamental question of fairness and civil rights."
"Civil union is not gay marriage," Hickenlooper said in a statement, adding the bill would only ensure "equal rights."
House Speaker Frank McNulty (R-Highlands Ranch) announced the committee selection late Monday morning and took the opportunity to blast Hickenlooper for using extra time on the issue of civil unions.
"We should not be spending time on divisive social issues when unemployment remains far too high, when far too many Coloradans remain out of work," McNulty said. "It's absolutely clear to me that Gov. Hickenlooper is reading straight from President Obama's campaign playbook trying to distract Coloradans and trying to distract Americans from their failed economic policies."
Hickenlooper responded, saying, "Greater attention to detail would serve [McNulty] well."
Hickenlooper says he sees civil unions as fundamentally different from gay marriage and that he is not coordinating his decisions in any way with the Obama campaign.
Civil union supporters pledged to continue the fight. There is not time to put the issue up for a vote of the people this November, but expect the legislation to return next session.
"Next year, we're going to push it again," Ferrandino said. "The speaker's choice was to kill these bills and kill civil unions in an undemocratic way."
Many opponents of the legislation who attended the committee hearing wore T-shirts with the words "loving all protecting marriage."
The legislature adjourned less than a week ago after the tense standoff that shut down business on the House floor.
Republicans hold a 33-32 voting advantage, but there was enough support for civil unions to pass, had it gone to full House for debate during the regular legislative session.
Democrats tried to force Republicans who control the calendar to bring the bill up for debate last Tuesday.
Republicans halted work, killing the bill and several others that needed a vote before a key deadline.
In an email to supporters, Rep. Dave Balmer (R-Centennial) condemned McNulty's actions to kill the civil unions legislation.
"I do not support abrogating the House Rules to pass or defeat any bill. The House Rules have their underpinnings in our State Constitution. I have served under three Speakers, and I've never seen the rules changed to advantage or disadvantage any specific bill. I never saw Speaker Romanoff bend the rules, so we must follow the Rules now. Bills should proceed to their normal committees of reference. The House Rules don't just belong to us (the 65 current Representatives). They belong to all Representatives who served before us and all those who will serve after us. More importantly, the House Rules belong to the People of Colorado," Balmer wrote in the email.
Balmer also reiterated he intended to vote "no" on civil unions.