Colorado Gov. John Hickenlooper plans to call a special session of the Legislature as soon as Friday to resolve a debate over civil unions that ended without a vote when the regular legislative session came to a close earlier this week.

Colorado voters banned gay marriage in 2006. But gay-rights advocates say the civil-unions legislation, which Hickenlooper, a Democrat, has said he will sign and which has already passed the Colorado Senate, has enough support to pass the House. It was blocked Tuesday by Republican House leaders.

A start date has not been set for the session, and it was not clear how long such a session could last. The Colorado Legislature’s last special session was in 2006; it was called to address immigration legislation, continued for five days and led to passage of a dozen measures.

Hickenlooper told the Denver Post that he would provide more information about the special session agenda Thursday, but indicated that it could include other bills that also died because they were not voted on before a Tuesday deadline. His office did not return calls or email Thursday.

“I think our goal is to make sure we do everything we can to try to make sure there is a fair, open debate on the floor of the House and the Senate,” the governor told the Denver Post, “that the issue gets discussed and that we allow people the chance to vote on it and that we move this entire state forward to make sure, as Martin Luther King said, ‘the arc of the moral universe is long, but it bends toward justice.’”

But even with the special session, Republican House Speaker Frank McNulty would not say whether civil-union legislation would get a vote in the House. In interviews, McNulty repeatedly referred to civil unions as “gay marriage,” even though supporters say the bill does not grant same-sex couples the same rights as married couples.

“If the governor wants to make this special session about gay marriage, then that’s his prerogative,” McNulty told the Post. His office did not return calls Thursday.

Rep. Mark Ferrandino, the Democrats’ leader in the House, was not optimistic that the bill would pass.

“Given the difference of a day, or a couple of days, I don’t know that I see a different outcome,” he told the Associated Press.

More than a dozen states allow either gay marriage or civil unions, including several that passed laws earlier this year.

[Updated: 10:49 a.m. May 9, 2012: A spokeswoman for Colorado Gov. John Hickenlooper said the special legislative session will start Monday, last at least three days and address civil unions and other issues. Hickenlooper is expected to release more information about the special session in a statement Thursday afternoon.]

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molly.hennessy-fiske@latimes.com