The measure has already cleared a committee and was scheduled for debate by the full chamber Wednesday. Both sides expected a long floor debate, though legislators could delay the debate if they wish.
The bill would allow couples to enter into a civil union so they could have several rights similar to married couples. Gay marriage is not possible here without changing the state Constitution; voters in 2006 changed the Colorado Constitution to ban same-sex marriage.
Supporters say the bill faces an uphill battle in the Republican House.