Supporters of liberalizing marijuana laws have scored big wins in recent years, as voters in both red and blue states have loosened marijuana laws. After November's elections, more than half of states will allow the use of marijuana for medical purposes, and eight states will allow marijuana for recreational purposes.

The legal marijuana industry is becoming a billion-dollar boon for businesses and investors and a reliable new source of revenue for cash-poor cities and states. Earlier this month, voters in Massachusetts, Maine, California and Nevada joined Washington, Colorado, Alaska, Oregon and the District of Columbia in legalizing marijuana for recreational use.

But marijuana remains illegal at the federal level, and pro-pot advocates have maintained an uneasy truce with the Justice Department under President Obama.

As attorney general, Sessions has a host of options for changing the federal government's posture toward marijuana.

He could follow precedent set by Holder and Lynch and let states chart their own path, or, on the other extreme, he could tell governors that any state that issues a license to permit marijuana sales would stand in violation of the Controlled Substances Act.

Sessions could revisit the Cole memo, the August 2013 memorandum written to federal prosecutors by then-deputy Attorney General James Cole that lays out the Justice Department's priorities in prosecuting drug cases. The Cole memo allowed prosecutors to skip cases in states that institute regulatory and enforcement systems to oversee marijuana sales.

To legal pot opponents, the Cole memo — and other steps the Obama Justice Department has taken — is an abdication of responsibility to implement federal law.

"We want to see federal law enforced. I think a clear letter asking states to stand down until Congress changes the law makes the most sense, and I think governors in these states would gladly oblige," said Kevin Sabet, who heads Smart Approaches to Marijuana, a group that opposes legalization.

The debate over marijuana legalization is a proxy, however imperfect, for the larger question of states' rights.

Legal marijuana backers say they hope Sessions and Trump let the states experiment as the founders intended.

"Voters in 28 states have chosen programs that shift cannabis from the criminal market to highly regulated, tax-paying businesses. Sen. Sessions has long advocated for state sovereignty, and we look forward to working with him to ensure that states' rights and voter choices on cannabis are respected," said Aaron Smith, who heads the National Cannabis Industry Association in Denver.

But opponents of marijuana liberalization say they see their own encouraging signs that the tide toward legalization may be turning.

"We've all wondered whether the Trump presidency would be 'states rights' or 'law and order' when it comes to drugs," Sabet wrote in an email. "The Sessions pick makes many of us think it may be the latter."

Even with Sessions overseeing the Justice Department, legal marijuana proponents are likely to continue pursuing liberalization through ballot measures and state legislatures.

Marijuana legalization measures are already circulating in Ohio, Texas, Mississippi and Missouri. Legislatures in states like New Jersey, Vermont, Delaware and Rhode Island are likely to take up marijuana legalization bills in upcoming legislative sessions.