WASHINGTON >> The District of Columbia defied threats from Congress and moved forward Thursday with legalizing possession of marijuana after a voter-approved initiative.

Despite last-minute maneuvers by Republican leaders in Congress and threats that city leaders could face prison time, D.C. Mayor Muriel Bowser said the city was implementing marijuana legalization as approved by voters. The new law took effect at 12:01 a.m.

Bowser, a Democrat, said the city’s plans haven’t changed despite a letter from two leading House Republicans warning of repercussions if the city moves forward with legalization.

"This is a major milestone on the road to ending marijuana prohibition in the United States," said Robert Capecchi of the Marijuana Policy Project, a group that advocates for legalization. "If the president can brew and drink beer in the White House, adults should be allowed to grow and consume a less harmful substance in their houses."

Congress has final say over the laws in the District of Columbia, and the two sides disagree about whether Congress acted quickly enough to block an initiative legalizing pot, which was approved by nearly two-thirds of city voters in November.

Rep. Jason Chaffetz, a Utah Republican who chairs the House Oversight Committee, urged Bowser in a letter late Tuesday to reconsider her plans to implement the initiative, saying that doing so would clearly violate federal law.

"Bullying the District of Columbia is not what his constituents expect, nor do ours," Bowser said. "We do disagree on a matter of law. There are reasonable ways to resolve that without us threatening him or he us."

The District becomes the first place east of the Mississippi River where recreational pot is legal. Alaska also legalized pot this week, joining Colorado and Washington state.

The initiative legalizes possession of up to 2 ounces for use at home, and people are also permitted to grow up to three mature plants. Smoking marijuana in public remains illegal, as does buying or selling the drug.

Congress approved language in December that appeared to block the initiative. District leaders argued it was enacted before Congress took action, even though it had yet to take effect. Chaffetz said that interpretation was wrong and that the mayor and other District employees would face possible prison time by moving forward.

"The penalties are severe, and we’re serious about this. Nobody’s wishing or wanting that to happen, but the law is clear," he said in an interview.

It would be up to the Justice Department to prosecute District officials, a scenario that appears unlikely. However, Congress could sue the city over its actions. House Republicans could also retaliate by pulling funding for other District programs.

The letter from Chaffetz and Rep. Mark Meadows of North Carolina warned that by spending money to change pot laws, Bowser and other District officials would violate the Anti-Deficiency Act, which prohibits spending of federal dollars that have not been appropriated. The committee also launched an investigation, demanding that the District turn over all documents detailing money that’s been spent and time that’s been put in by city employees to implement the initiative.

No one has ever been convicted of violating the Anti-Deficiency Act, although government employees have been punished administratively for violations.

Jamie Raskin, a constitutional law professor at American University, characterized the threat of criminal prosecution as "a lot of huffing and puffing on Capitol Hill."

"The real power Congress has is the power of the purse," said Raskin, who’s also a Democratic Maryland state senator. "They can exact their retribution financially against the District."

Congress hasn’t struck down a specific city law in 25 years. Instead, members often add language to critical pieces of federal legislation to undo city policies they don’t like. The language on pot was included in a spending bill that was needed to keep the government running.

The city has already decriminalized possession, and under legalization, police officers would no longer issue $25 civil fines for people caught with pot.

Bowser said that by implementing the initiative, she was simply doing her job, and she said she hoped she could work more productively with Congressional Republicans on other issues.

"We would encourage the Congress to not be so concerned about overturning what seven out of 10 voters said should be the law in the District of Columbia," she said.