House Republicans on Wednesday blocked funding for a new D.C. law that would eliminate the threat of jail time for pot possession, leaving the fate of one of the country’s most liberal decriminalization efforts unclear.

The D.C. law, which is to take effect next month, makes possession in the nation’s capital punishable by a fine of just $25.

A spokesman for Mayor Vincent C. Gray (D) said the city would proceed with implementing the law even as its authority to mete out the smaller penalties beyond October remains up in the air.

Gray’s office also warned that as the House Republican amendment was drafted, the city needs to determine if the action could force the city to shut down its entire medical marijuana program, which started last year. Marijuana advocates, meanwhile, were pushing a theory that the House Republicans’ actions could leave the city with no enforceable marijuana law, effectively legalizing possession. The D.C. Attorney General’s Office said it was reviewing that possibility and other potential fallout from the House’s vote.

The interference from House Republicans, however, was broadly seen as another defeat for home rule in the District — and the ability of its Democratic mayor and left-leaning council to self-govern. In March, the council voted overwhelmingly to eliminate jail time for possession, calling it necessary to combat deep racial disparities in drug arrests in the city. Studies have shown that use among teenagers varies little by race but that African Americans account for nine of 10 possession arrests in the nation’s capital, contributing to an estimated 10 percent of residents who have criminal records and trouble finding jobs.

The effort to keep the District from loosening its marijuana laws was led by Rep. Andy Harris, Maryland’s only Republican in Congress.

In April, Maryland also joined the third of states that have passed similar laws eliminating jail time for pot possession. But Harris argued that the D.C. law was “bad policy” assessing a fine of just $25 — a fraction of the $100 fine in Maryland, which escalates rapidly with repeat violations.

Harris, a doctor who represents the Eastern Shore, also visibly swayed fellow Republicans by noting that the District’s law has no drug-treatment component, even for minors, and that the fine for a young teen who is caught with a joint would be half that of the city’s $50 ticket for underage smoking of a cigarette.

“No referral for help? Not even for a 14- or 15-year-old. That’s just plain bad policy.” Harris told members of the House Appropriations Committee. “This is the opportunity to stop that bad policy from moving forward.”

Harris’s amendment, or “rider” on a a multibillion-dollar spending bill, would preclude the District from spending any money “to enact or carry out any law, rule, or regulation to legalize or otherwise reduce penalties associated with the possession, use, or distribution of any” federal controlled substance.

It was one of several riders on D.C. spending House Republicans passed Wednesday, the others including a prohibition on D.C. spending its own money on abortions for poor residents and one cutting funds for D.C. police officers to drive their cruisers to and from their homes if they live outside the city.

The riders were derided by House Democrats from New York to California as infringing on the District’s right to self-governance.

Rep. José E. Serrano (D-N.Y.) called Harris’s amendment a misplaced effort by Republicans to appear tough on drugs.

Serrano, whose family is from Puerto Rico, also said that Republicans’ interference amounted to D.C. “colonialism,” with representatives from red states imposing their will on the city’s Democratic majority.

“The D.C. voters elected people. They made the decision, and it seems to me that we ought to respect that,” added Rep. James P. Moran (D-Va.). “It just doesn’t seem right that the Eastern Shore of Maryland can reach over into D.C. and make laws for D.C. It’s not the way this country is supposed to function.”

One Democrat, Rep. Henry Cuellar of Texas, sided with Republicans in the 28-to-21 vote and Harris seized on the defection, calling his amendment a bipartisan success.

In a brief interview, Harris said his bill was not intended to upend the District's medical marijuana program and was drafted carefully to not do so.

“This is not about medical marijuana. This is about decriminalization and the effect of that on the youth of this country. And in this case, the youth of D.C.”

Harris’s amendment and the other riders will be included in the budget that goes to the full House and then to negotiators, who will work out a compromise with an as-yet unfinished Senate spending plan, which is almost assured to include no similar restrictions on the District.

The House hearing was the last expected hurdle for the District’s marijuana law, which is nearing the end of a 60-day congressional review period. Unless Congress passes a measure halting the law — and the president signs that — it will take effect next month.

Under the law, possession would draw a civil fine of $25 — akin to a parking ticket. That would be the smallest penalty outside any state except Alaska, which has no fine, and Colorado and Washington, which have legalized marijuana for recreational use. Smoking pot in public could draw a penalty of 60 days in jail, like an arrest for an open container of alcohol.

The District law has drawn praise from marijuana advocates and civil rights groups, and on Wednesday, their representatives lashed out at Harris.

“That Rep. Harris is picking on a majority black district and no other jurisdiction with marijuana decriminalization is very telling,” Bill Piper, director of national affairs with the Drug Policy Alliance, said in a statement. “His own state has decriminalized marijuana, but he’s not interfering with it.”

Tom Angell, chairman of the Marijuana Majority, cast the move by Harris and House Republicans as out of step with voters.

A Washington Post poll this year found that D.C. voters, by about 2 to 1, support legalizing small amounts of marijuana for personal use.

A band of marijuana advocates is working toward a deadline next month to gather enough signatures to put a measure before voters in November to legalize possession, including home cultivation of the plant.

Such a law would also end up back in Congress, where House Republicans, at least on Wednesday, were ready to rewrite it as they saw fit.

“The federal government has a unique relationship with the city of Washington, D.C.,” said Rep. Ander Crenshaw (R-Fla.). “We, the federal government, pay for the city’s courts, we pay to house their prisoners, and I think we pay for a few other things . . . so the Constitution says we have the power to exercise — in all cases — over this District. And that is what we do.”