Wednesday's vote brings gay couples a step closer to a showdown in the Republican-controlled House, which is likely to determine the bill's outcome. A Senate committee approved the legislation on a 5-2 vote with one Republican joining the Democrats on the committee.
It's expected to clear the full Senate for votes in the House.
A nearly identical bill failed last year in the House Judiciary Committee, which still has the same members today and is expected to handle the bill again.
If it fails at that same point, supporters vow to keep on trying.
"Every year this gains ground," Sen. Pat Steadman (D-Denver,) who sponsored Senate Bill 2, said. "One of these days it's going to cross the finish line. It's coming soon."
Opponents complain that civil unions would eventually lead to the legalization of same-sex marriages in Colorado.
"This particular legislation has been used as a springboard to same-sex marriage in virtually every place that it's been instituted," Brian Raum, an attorney with the Alliance Defense Fund, said, "either through the court system or legislatively."
Civil unions are as far as the lawmakers can go because of the voter-approved amendment to Colorado's constitution banning same-sex marriage.
Those arguing in opposition complain that civil unions are too similar to marriage because they would grant civil partners many of the same rights as married couples.
That drew the ire of the bill's sponsor.
"Which one of these things should we subtract to the list to make this far enough to the back of the bus so that it's acceptable?" Steadman asked. "I don't think any of them should be subtracted to the list because I don't want anybody to sit at the back of the bus."
More than a dozen states allow either civil unions or same-sex marriage.
The governor of Washington state signed a bill legalizing gay marriage Monday, and the New Jersey Senate passed a similar measure the same day. Both face challenges.
Civil union laws also took effect this month in Hawaii and Delaware.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)