The measure is expected to pass the Democratic legislature and be approved by the Democratic governor, The Denver Post reported. The Senate Judiciary Committee approved it on a 3-2 party line vote.
Religious groups testified against civil unions and charged that the bill would erode religious freedom. Carrie Gordon Earll of CitizenLink, an organization with ties to Focus on the Family, suggested its backers' ultimate goal is to redefine marriage.
At least one Republican group supported the bill. Mario Nicolais of Coloradans for Freedom, compared expanding the rights of gays to the battle to end slavery depicted in "Lincoln."
"I know many people who are afraid of the so-called homosexual agenda," he said. "The so-called homosexual agenda is nothing more than a desire to participate in the American dream the rest of us are already afforded."
In 2006, Colorado voters approved a referendum defining marriage as being between one man and one woman.