DENVER — Marijuana legalization could be going back to the ballot in Colorado — a prospect that has pot legalization activists furious.

State lawmakers are mulling a bill to tax the newly legal drug more than 30 percent. Some lawmakers want to add a caveat to the tax proposal — that recreational pot won't be legal anymore unless voters approve the taxes this fall.

Marijuana activists immediately blasted the proposal as a backhanded effort to repeal the pot vote, in which 55 percent of Coloradans chose to flout federal drug law and declare pot legal in small amounts for adults over 21.

"It's clear that the intent ... is to prevent marijuana from being legal and being regulated and being controlled," said Mason Tvert, who led last year's campaign to add recreational pot to the state constitution, which has allowed medical marijuana since 2000.

The source of the outrage is a draft bill floating around the Capitol late this week that would open the door to repeal.

The bill would revise the ballot question on pot taxes to add that recreational pot in the state constitution should be repealed if voters don't approve 15 percent excise taxes on retail pot and a new 15 percent marijuana sales tax. Those would be in addition to regular state and local sales taxes.

"The whole purpose of it was to raise money for education and so forth, so if there's no money, we shouldn't have marijuana," said Sen. Larry Crowder, R-Alamosa.

A volunteer group that has been critical of proposed marijuana regulations, Smart Colorado, praised the effort to get rid of recreational pot without approval of the taxes.

A spokesman for the group, Eric Anderson, said in a statement that marijuana activists "sold the ballot issue to Colorado voters as a way to pay for state priorities like education, but increasingly it's looking like it could be a net drain on the state budget."

The marijuana measure approved last year won more votes than President Barack Obama, who carried the state. The pot measure directed lawmakers to come back to the ballot with a tax proposal, with much of the money going to school construction. But because of Colorado's Byzantine tax laws, the recreational pot taxes can't be levied until voters again sign off on them.

In Washington state, the only other place where voters last year approved recreational pot, the ballot measure set taxes at 75 percent, settling the question. Both states are still waiting to find out whether the federal government plans to sue to block retail sales of the drug, set to begin next year.

The Colorado repeal effort wouldn't apply to medical marijuana, which voters approved in 2000.

Lawmakers from both parties have expressed worry this year that Colorado won't be able to afford to give recreational pot the kind of intense oversight and regulation many expect. From labeling and potency standards to making sure pot taxes are collected, the regulatory scheme under consideration in Colorado wouldn't be cheap.

Even backers of the 30 percent taxes aren't sure what they would raise.

"We have to hope and expect for the best but prepare for the worst," said Rep. Jonathan Singer, D-Longmont, selling the tax proposal to a House committee Thursday.

The state House was planning a debate Friday on the tax ballot question. The repeal provision, if it appears, would come later, likely when the pot tax shifts to the Senate.

Some lawmakers said Friday they doubt lawmakers would send pot legalization back to voters this year.

"That's almost like saying to voters, 'Vote for this, or else,'" said Sen. Cheri Jahn, D-Wheat Ridge. "I don't think you threaten voters like that. When over 55 percent of the people vote for something, I think we have to respect that."

Marijuana repeal debate could dominate the Legislature's closing days. The path to repeal would be uncertain, but some lawmakers say it's only fair to ask again if voters are willing to legalize pot and risk federal intervention in exchange for a tax windfall projected to exceed $100 million a year.

"I think that's why the people supported it," Crowder said.

-- The Associated Press