The U.S. state of Colorado has had a system of direct voting since gaining statehood in 1876. Citizens and the Colorado General Assembly both have the ability to place new legislation, those recently passed by the General Assembly, and constitutional amendments on the ballot for a popular vote. Colorado has three types of ballot measures that can be voted on in a statewide election: initiatives, referendums, and legislatively referred measures. In order to be placed on the ballot, supporters of a measure must gather signatures from registered voters. From 1877 to 1910, the only ballot measures allowed were legislatively referred measures. In 1910, Referendum 3 was placed on the ballot by the General Assembly and passed, creating a citizen-led process for initiatives and referendums. The first successful citizen-initiated measures were passed in 1912.

Since that time, ballot measures have played a major role in Colorado politics. After Denver was awarded the hosting rights to the 1976 Winter Olympics, citizens moved to block funding the games with a ballot measure in 1972. A 1990 ballot measure instituting term limits for many elected officials helped galvanize a nationwide movement for term limits, and 2000's Amendment 20 legalized the medical use of marijuana. That measure was followed by full decriminalization in 2012 and the decriminalization of psilocybin mushrooms in 2022.

The 1876 Constitution of Colorado included procedures for the General Assembly to place measures on the ballot in a statewide election. In the 1890s, a grassroots movement to increase citizen power began, culminating in a special session of the legislature to discuss initiative and referendums in 1910. That session resulted in 1910's Referendum 3, which passed with over 76% of the vote and created a citizen-initiated process.^[1] From 1877 to 2016, constitutional amendments required only a simple majority to pass. In 2016, Amendment 71 passed and raised the threshold to 55%.^[2][3] In 2020, as part of his administration's response to the COVID-19 pandemic, Governor Jared Polis issued an emergency rule allowing petition signature gatherers to do so via email and mail, rather than in-person efforts. The change, while upheld by the Denver District Court, was overturned by the Colorado Supreme Court later that year.^[4]

After the 1876 Constitution was adopted, a legislatively referred constitutional amendment which would have granted women the right to vote was placed on the ballot for the 1877 election.^[5]: 92 The measure failed, with over two-thirds of voters against it.^[6] Henry Blackwell, a founder of the American Woman Suffrage Association, summed up the unsuccessful campaign by saying "Woman Suffrage can never be carried by a popular vote without a political party behind it".^[7] Blackwell was proven correct in 1893 when, in part due to gains made by the Colorado People's Party in the General Assembly, voters supported a women's suffrage ballot measure by a 55-45 margin.^{[8][5]: 124–158}

In 1970, the International Olympic Committee granted Denver hosting rights for the 1976 Winter Olympics.^[9] Governor John Love claimed that the games would cost taxpayers only $5,000,000. Activists quickly noted a wide variety of issues with the state's cost estimate, however, including a lack of transportation infrastructure, no planning for the Olympic Village, and issues with planned events sites.^[10] A petition for 1972's Measure 8 quickly reached the necessary 51,000 signatures and 60% of Coloradans voted to prohibit the state from funding the Olympics. Later estimates found that the cost for Denver to host the games would have been $92,000,000, over 18 times the state's estimate.^[11] Richard Lamm, who was a leader in the local anti-Olympics movement, would later parlay his fame from the measure into three terms as Governor.^[12]

The 1984 ballot included Amendment 3, which barred the use of state funds for abortion services. The measure passed by less than one percentage point and gave Colorado the distinction of being both the first state to decriminalize abortion, having done so in 1967, and the first state to prohibit the government from funding it.^[13] 1990's Amendment 5 has been credited by the Initiative & Referendum Institute as having started the term limits movement in the United States and was followed by similar initiatives in 1994 and 1996. Colorado's efforts were unique because they placed term limits on members of Congress in addition to state-level officials.^[1] The term-limits movement resulted in the Supreme Court case U.S. Term Limits, Inc. v. Thornton (1995), which determined that states could not place restrictions on congresspeople beyond the constitutional requirements.^[14][15]

In 2000, Amendment 20 passed with 54% of the vote and legalized the medical use of marijuana in the state.^[16] It was followed by 2012's Amendment 64, which passed by similar margins and legalized the recreational use of marijuana.^[17] The Economist described the vote as "an electoral first not only for America but for the world."^[18] Colorado continued this trend of loosening drug policy in 2022 when voters passed Proposition 122 and legalized the use of psilocybin mushrooms in designated "healing centers".^[19]

• Initiatives and referendums in the United States
• History of Colorado
• Law of Colorado