Colorado Attorney General John Suthers, an ardent opponent of marijuana legalization, said Wednesday he would respect the will of the voters on Amendment 64 but cast doubt on the measure's longterm success.

The measure — which legalizes possession of up to an ounce of marijuana and allows for a commercial, recreational marijuana industry — passed on Tuesday and will take effect as soon as the final vote is certified sometime before the end of the year.

"Despite my strongly held belief that the 'legalization' of marijuana on a state level is very bad public policy, voters can be assured that the Attorney General's Office will move forward in assisting the pertinent executive branch agencies to implement this new provision in the Colorado Constitution," Suthers wrote in a statement.

But Suthers cast doubt on the longterm legality of Amendment 64, saying, "the ability of the federal government to criminally sanction possession, use and distribution of marijuana, even if grown, distributed and used in a single state, was recognized by the U.S. Supreme Court ."

"Therefore, absent action by Congress, Coloradans should not expect to see successful legal challenges to the ability of the federal government to enforce its marijuana laws in Colorado," Suthers wrote.

He urged the U.S. Department of Justice — which has been considering lawsuits in Colorado and in Washington, where a legalization measure also passed Tuesday, to block the new marijuana laws — to articulate its response "as soon as possible."

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold