DENVER — A pitch to shore up Colorado’s Medicaid program and schools by ramping up taxes on the state’s wealthiest residents will likely go to voters next year under a proposal set to be announced Wednesday morning by a coalition of progressive groups.

The proposed ballot measure would end Colorado’s decades-old flat income tax policy and require those making more money to pay higher rates. The measure, if approved by voters in November 2026, would cut the effective income tax rate for individuals making less than $506,000 — while raising the effective tax rate for those above that threshold.

The changes would raise another $2.3 billion per year for the state, according to an analysis by the initiative’s backers. They would also include breaking a key piece of the Taxpayers’ Bill of Rights, or TABOR, the longstanding state constitutional amendment that governs state tax policy.

TABOR, among other things, requires the state to have the same income tax rate for all residents, regardless of income. Colorado is one of 15 states with a flat tax, while 27 others plus the District of Columbia tax higher-income residents at a higher rate than lower-income residents pay.

Prior to TABOR’s adoption in 1992, Colorado used a graduated income tax system.

In effect, that flat tax has foisted too much of a burden on the poorest Coloradans as they try to balance their own needs with the needs of their community, said Chris deGruy Kennedy, a former state lawmaker who is president of the progressive Bell Policy Center.

“We think it’s been kind of a false choice between, ‘Are you willing to pay more, as a regular working person, in order to have a better school system?’ — when it’s really about, ‘Are you willing to make the tax system more fair (and) to make the wealthiest pay their fair share, so that we can have the health care, education and child care investments that make Colorado the kind of state that supports opportunity for all?’ ” DeGruy Kennedy said.

The graduated income tax proposal is backed by a dozen liberal groups, with the coalition called Protect Colorado’s Future. The coalition includes New Era Colorado, the Colorado Children’s Campaign, Great Education Colorado, the Colorado Organization for Latina Opportunity and Reproductive Rights, and more.

The measure would enact a new state law and amend the state Constitution — though, because it only seeks to remove and not to add language there, it would need only a simple majority to pass, backers say. If approved by the title board in coming months, backers would still need to gather valid signatures to secure a place on the 2026 ballot.

While the coalition is optimistic, Colorado voters rejected the last attempt to change to a graduated income tax. In 2018, nearly 54% of voters said no to Amendment 73, which would have raised money for the education system by increasing taxes on higher earners. Because it would have added to the state constitution, though, that measure would have needed 55% support to pass.

Backers of the new proposal are introducing a handful of differing versions to the state title board, which governs the ballot initiative process, for review.

One of the variations would require that tax money collected at the higher rates goes specifically to public education, health care, child care and public safety — which is broadly defined to include anti-poverty programs, substance abuse recovery and natural disaster resilience.

Another would swap in workforce development for public safety, while the third variation would allow unrestricted use of tax collections.

Filing multiple versions of substantively similar proposals is a common practice. DeGruy Kennedy says the backers want to keep their options open before settling on a proposal to put on the ballot.

The basic premise remains the same, backers say: Almost every Coloradan would see tax savings of a few hundred dollars under a lower rate, or pay what they do now. But people making $1 million a year would pay almost $18,000 more, with the rate steadily increasing until those making $10 million per year paying some $476,000 more in state income taxes.

DeGruy Kennedy framed the change as raising rates on people who’ve benefited the most from economic growth over the past several decades.

The proposal also comes as the state braces for upcoming changes to federal spending as part of a law recently signed by President Donald Trump. The new law, known as the One Big Beautiful Bill Act, will shift more costs for Medicaid onto states and potentially punch a massive recurring hole in Colorado’s budget — in addition to the immediate state budget deficit it caused this summer — unless the state either cuts programs and spending elsewhere or, somehow, raises taxes.

Even without that new challenge, Colorado has been living in a state of “false austerity,” said Kathy White, the president of the progressive Colorado Fiscal Institute. Earlier this year, spiking Medicaid costs helped drive $1.2 billion in cuts to other parts of the state budget because of TABOR’s total spending cap — before any federal changes took effect.

“I just hope it will be clear to people that we’re really thinking about the future,” said White, whose group is part of the coalition. “We believe in Colorado, we believe in the future of Colorado. We are handing our kids a mess, and we’re taking away all of the tools they will need to fix it.”

The proposal is certain to raise the hackles of many conservatives across the state and of people opposed to income taxes, and it may fuel another ballot-box battle.

Advance Colorado, a leading conservative advocacy organization, has already filed several initiatives proposing to cut spending or otherwise limit the state government’s ability to raise money without voter approval.

The emerging measure is also likely to face opposition from Gov. Jared Polis, a Democrat who has called for the elimination of the income tax. He’s supported previous efforts to lower the tax rate in the state. Last year, Polis negotiated a conditional income tax reduction that he required to earn his support for a program intended to cut child poverty.

In an August interview about eliminating business-friendly tax incentives, he told The Denver Post that he would support additional changes from the Legislature next year only if lawmakers agreed to another income tax cut.

Polis’ office did not return a request for comment Tuesday on the ballot measure effort.

How the measure would change income tax rates

Colorado now has a 4.4% income tax rate on residents, regardless of income, though it has been lowered in years when tax collections exceed the cap set by the Taxpayer’s Bill of Rights. Here are examples of how the newly proposed ballot measures would change the effective tax rate depending on income, with rates rising with income:

—$100,000 or less: 4.2%

—$250,000: 4.3%

—$500,000: 4.4%

—$700,000: 5.3%

—$1 million: 6.2%

—$1.5 million: 7.3%

—$3 million: 8.4%

—$5 million: 8.8%

—$10 million-plus: 9.2%

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(Source: Protect Colorado’s Future coalition)

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