Amendment 73 looks like it will fall short of the 55 percent approval it needs to pass.
Residents making more than $150,000 annually would have seen their tax rate increase incrementally based on a new bracket system, which would have increased state coffers by $1.6 billion annually.
The amendment would have created the "Quality Public Education Fund," which would have been used to pay for additional programs in preschool through 12th grade in Colorado.
It would focus on increasing funding for special education, preschool, English language classes and gifted programs.
According to the organization Great Schools, Thriving Communities, which supported the measure, it would take an additional $830 million to bring education funding back to levels seen prior to cuts during the Great Recession.
Other groups supporting the amendment included the Colorado AFL-CIO, Sen. Bernie Sanders, many school districts, the Colorado Education Association and the League of Women Voters.
Many groups in Colorado opposed the measure, including the Denver Metro Chamber of Commerce, the TABOR Foundation, several trade groups and Ready Colorado, a conservative group that supports school choice.
Phil Pfiffner, board chairman for the TABOR Foundation, told Colorado Watchdog in August that the measure was greedy and unreasonable.
Opponents also said they didn't like how the amendment would just throw money at the problem instead of attempt to reform the public school system. Colorado Watchdog, for example, said reforming the school system would likely guarantee better student performance, while a blank check might not.
On Wednesday morning, the Denver Public School District, the state's largest school district, issued a statement about the failure of Amendment 73.
"This ballot measure would have provided our schools roughly $150 million in additional funding for teacher compensation, supporting higher needs students and expanding Early Childhood Education opportunities," the statement said in part. "In Colorado, we fund our schools an estimated $2,100 per student less than the national average. We know our kids deserve better than that. Our state needs to increase our investment in education, and all of our voices play a vital role in this effort."