WASHINGTON, DC - NOV 04: Initiative 71 supporters celebrate as early returns show the District’s pro-pot law cruising to an overwhelming victory on Nov. 4. (Michael S. Williamson/The Washington Post)

The District of Columbia defied the new Republican Congress on Tuesday, challenging the House and Senate to either block or let stand a voter-approved ballot measure to legalize marijuana in the nation’s capital.

D.C. Council Chairman Phil Mendelson (D) sent the measure to Capitol Hill, starting the clock on a 30-day review window that Congress has used only three times in 40 years to quash a local D.C. law.

If Congress or President Obama do not act to block it, the ballot measure permitting possession of up to two ounces and home cultivation of pot could become law in the District as early as March.

Another possible scenario, however, is that Tuesday’s move would launch a rocky year of legal battles — and thrust a final decision on pot legalization in the city into the hands of federal courts.

Mendelson took the provocative step of sending the marijuana measure to Congress despite a federal spending bill passed last month and signed by the president that explicitly prohibits the District from enacting new laws to reduce penalties for drug possession.

Conservative House Republicans boasted at the time that the paragraph tucked into the 900-page, $1 trillion spending bill halted the city from following Colorado and Washington state into a closely watched experiment to legalize marijuana.

“That issue has come and gone,” said Rep. Jason Chaffetz last month. The Utah Republican is the new chairman of the powerful committee with oversight of D.C. issues and is an outspoken opponent of marijuana legalization. In a December interview, Chaffetz warned that any attempt by the District to move forward on legalization would be “ill-advised and fruitless.”

But Mendelson — backed by the new mayor, Muriel E. Bowser (D) — vowed to carry out the will of the 7 in 10 D.C. voters who supported Initiative 71.

Mendelson, Bowser and Del. Eleanor Holmes Norton (D), the District’s nonvoting House member, said they have a solid legal basis for pressing forward with implementing the ballot measure. The spending bill, they said, prohibits the District from using taxpayer funds to “enact” new laws that loosen drug restrictions — but not to implement official acts.

Under D.C. law, ballot measures are considered official acts — equal to bills passed by the D.C. Council — once election results are certified. The city’s Nov. 4 election results were certified Dec. 3, a week before Congress passed the spending bill that included the restriction on any new laws.

In letters dated Tuesday and addressed to Vice President Biden and House Speaker John A. Boehner (R-Ohio), Mendelson noted the dates, saying the pot law was a done deal before the federal spending bill was passed.

In an interview this week, Mendelson played down the issue, saying there was “nothing special” about transmitting the measure despite warnings from Republicans in Congress. He also stressed that he wasn’t seeking a confrontation with Congress.

“I have no choice,” Mendelson said. “The law says that I must transmit the measure. That is all I am doing.”

The business-as-usual stance is key because in 1998, Congress similarly restricted the expenditure of taxpayer funds in order to block a ballot measure legalizing medical marijuana. Once Congress acted, D.C. election officials declined to spend even $1.64 to finish counting ballots, fearing retribution from Congress. The city’s medical marijuana law ultimately stalled for more than a decade.

To underscore that the District was making no special expenditure this week to take the final steps to make Initiative 71 law, Mendelson did little to draw attention to Tuesday’s effort to get the measure to Congress. Legislative staffers bundled it in manila envelopes with 20 other acts of the D.C. Council that were due before Congress for a standard 30-day review. The city also did not alter the monthly schedule to send the package to Congress. Legislative staffers said they could not single out any particular cost for the envelope, printing or other packaging of the ballot measure.

In a statement after the initiative reached Congress, Rep. Andy Harris (R-Md.), a leading opponent of marijuana legalization, cautioned that the District was attempting to subvert the intent of Congress and to rewrite the definition of when a bill becomes law — in the District’s case, always following congressional review.

“Despite attempts to misconstrue the language of the omnibus bill, it is clear that if D.C. chooses to proceed with enactment of marijuana legalization, they will be in violation of the law,” Harris said.

The District faces a host of other challenges should its effort to implement the ballot measure succeed.

Mendelson said there is no doubt that the federal prohibition on looser drug laws will prevent the District from adopting a system for legal sales and taxation of marijuana, like in Colorado and Washington state.

Before she pledged to fight the congressional interference, Bowser, the new mayor, said she feared that moving forward with one but not the other could lead to open-air drug markets and tie the hands of police.