Denver Pre-election polls indicated a cakewalk for Amendment 46, the initiative to end race- and gender-based affirmative action programs.
But a sizable ground game helped in part by a huge effort from other campaigns to turn out voters made the so-called Colorado Civil Rights Initiative much too close to call on election night.
The amendment was losing slightly as returns came in.
"Four years ago today, I ran for state Senate and felt I had all the momentum going into it but I lost," said Jessica Corry, leader of the pro-46 campaign. "So I don't take anything for granted."
The amendment would prohibit discrimination and preferential treatment by government on the basis of race or gender, effectively ending public programs aimed at increasing minority or female participation in contracting, education or hiring by governments.
An identical measure passed by comfortable margins in California 10 years ago, and in Washington and in Michigan. It also passed in Nebraska on Tuesday.
If the trend holds up, Colorado would be the first state to reject the initiative that is the baby of California millionaire entrepreneur Ward Connerly.
Although its backers called it a civil rights initiative, it actually would eliminate race- and gender-based affirmative action programs, and that's why the traditional civil rights community lined up against it.
"What we learned from those other states that had it was that our biggest issue to push was the deceptive nature of the amendment," said Melissa Hart, a University of Colorado law professor who led the No on 46 campaign.
Amendment 46 galvanized civil rights and equal opportunity advocates into a single purpose - defeating it. And they used grass-roots organizing, as well as attaching their efforts to large get-out-the-vote drives by other groups.
In addition, Hart said with so much advertising this year on voting "No" on many of Colorado's numerous ballot measures, her campaign might have benefitted from the spillover.
"I think we absolutely did everything we could have done," said Hart.