Congress's massive spending bill will likely block a voter-approved marijuana legalization initiative in Washington, DC. The spending bill "prohibits both federal and local funds from being used to implement a referendum legalizing recreational marijuana use in the District," according to a summary from the House Appropriations Committee.

The White House could veto the spending bill, but that's widely considered unlikely since it would result in a government shutdown.

DC's voter-approved initiative wouldn't legalize pot sales

DC's marijuana legalization initiative obtained more than 69 percent of the vote in November. The initiative would allow adults 21 and older to possess up to two ounces of marijuana, grow up to six plants, and give marijuana to other adults 21 and older. It wouldn't legalize, regulate, or tax sales, because voter initiatives in DC can't have a direct impact on the local budget.

Congress can stop any of DC's laws

Through the Home Rule Act of 1973, Washington, DC, can elect a sitting local government composed of a council, mayor, and other local agencies. But the Home Rule Act also made it so each bill passed by the local government requires congressional approval. And Congress can block DC's laws through budgetary requirements, as they are doing with the legalization initiative.

(Read more: 6 questions about Washington, DC, statehood you were too disenfranchised to ask.)

Congress previously used this authority to block DC from implementing a medical marijuana law for nearly 12 years. Federal lawmakers have also prevented DC from using local tax dollars to fund abortion services and life-saving clean needle exchange programs.

The spending bill blocks funds to implement legalization



The budget bill blocks DC from using local and federal funds to carry out its legalization measure.

At first glance, this might seem like a weird approach. DC's legalization initiative costs nothing; it actually saves the district money to not enforce laws against marijuana possession. The ballot measure actually couldn't cost money in the first place, since DC ballot initiatives, by law, can't have a direct impact on the local budget.

But the budget bill would prohibit DC Council from spending its time and resources to approve the legalization initiative and send it to Congress. Under federal law, that's a necessary step for legalization to take effect.

Decriminalization remains in place in DC

Although the bill blocks the implementation of legalization in DC, a marijuana decriminalization law will remain in place since it already took effect in July.

The decriminalization law removes criminal penalties on marijuana possession but keeps a $25 civil fine and prohibits public use. The legalization initiative would have gone one step further, repealing the civil fine and also allowing DC residents to grow and gift their own marijuana.

Under a White House veto threat, House Republicans unsuccessfully attempted to block DC's decriminalization law before it took effect in July. But since the appropriations bill is widely considered a must-pass to avoid a government shutdown, it's very unlikely DC's legalization initiative will be able to rely on the White House for similar support this time around.

Update: This story was updated to reflect the final language of the budget bill.