David Flaherty, a Colorado-based GOP consultant at Magellan Strategies, said the Justice Department’s decision could lead to a “major backlash and a spike in younger voters” if it disrupts the current system in Colorado. “Folks that are 44 and under here in Colorado are much more comfortable with the legalization of marijuana,” he said.

Flaherty said Colorado Republicans must navigate a GOP primary electorate with as many as half of voters age 65 and older, many of whom want to make marijuana illegal again.

Popular Opinion

Marijuana legalization has grown in popularity: 64 percent of Americans favor it, according to an October 2017 Gallup poll. Support was 57 percent to 37 percent in a Pew Research survey released a year earlier — including a remarkable 71 percent of millennials, currently the largest group of eligible voters in the country.

"Sessions’s move just adds another weight to the ankles of vulnerable House Republicans in places like California and Colorado," said Brian Fallon, a former spokesman for the Senate Democratic leadership and Hillary Clinton’s 2016 campaign. "Given the support for decriminalization across political parties, and especially among young voters, this was an issue that progressives already should have been considering for state ballot measures. That is even truer now."

Gardner -- who doesn’t face re-election until 2020-- isn’t being shy. He vowed to block Justice Department nominees from being confirmed unless Sessions reverses course. Sessions, a former Alabama senator, said in an April 2016 hearing: "Good people don’t smoke marijuana."

Democrats have struggled for years to turn out voters under 30, who tend to lean left, but there’s some evidence that the issue of marijuana can help. That can be seen in 2012 exit polls in states where pot legalization was on the ballot. The share of the electorate age 18-29 jumped 6 points in Colorado, 5 points in Oregon and 12 points in Washington compared to 2008.

"It’s time for anyone who cares about this issue to mobilize and push back strongly against this decision," said Representative Earl Blumenauer of Oregon, a Democrat who is co-chairman of the Congressional Cannabis Caucus.

Pot’s Potency

Some Democrats are skeptical about whether the issue will sweep their candidates into office.