WASHINGTON — Possession of marijuana becomes legal in the nation’s capital early Thursday with Mayor Muriel Bowser, defying a threat from Congress that she and other city leaders could face prison time for implementing a voter-approved initiative.

Bowser, a Democrat, said on Wedneday the city’s plans would not change despite a letter from two leading House Republicans warning of repercussions if legalization took effect as scheduled at 12:01 a.m. Thursday.

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.

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