This year Californians will vote on a ballot proposition that would legalize the sale and possession of small amounts of marijuana for personal use. Democrats around the country will be watching the vote closely to see if it could hold the key to the 2012 elections.

It’s not that the Democratic Party necessarily supports legalizing marijuana. In fact, neither Sen. Barbara Boxer (D-CA), who is facing a tough battle for reelection, nor Attorney General Jerry Brown, the Democratic candidate for governor, support Proposition 19. Democrats are reluctant to support legalization, for fear for of the moral stigma associated with drug use.

There are good reasons to support legalization, of course. Marijuana, like any drug, can be abused, but it’s hardly more dangerous than alcohol. Just as with alcohol, it probably makes more sense to regulate marijuana's use than it does to prohibit it entirely. Criminalizing marijuana wastes police and legal resources on what is at worst a minor infraction. Legalizing marijuana would eliminate the black market for the drug. That which would reduce drug-related violence in the U.S., and substantially weaken the Mexican cartels that have are tearing apart our neighbor to the south. Marijuana is practically already legal in California as it is. Governor Arnold Schwarzenegger recently signed a bill reducing the possession of less than an ounce from a misdemeanor to a mere infraction—the equivalent of a speeding ticket. Many police departments choose not to use their limited resources prosecutor minor offenders anyway. And it’s fairly easy to get a card authorizing you to use marijuana for a wide variety of medical reasons.

But none of that’s why legalization may be good politics for Democrats. Even though many prominent Democrats don’t support legalization, the people who are in favor of legalization tend to vote Democratic anyway. If the prospect of voting for legalization gets people who are likely to vote to Democratic to the polls, that’s great for the Democrats, whether or not the measure passes. Peter Wallsten reports in The Wall Street Journal this week that pollsters are seeing just such a “coattail effect” among voters under 30 in California. It’s easy to joke that supporters of legalization will be too stoned to come to the polls, but it's not just regular users who support legalizing the drug. Polls show that close to half the country supports legalization. Most of those people are Democrats. Just as ballot measures banning gay marriage got social conservatives to the polls in 2004—and helped hand George W. Bush the election—Proposition 19 may get liberals to the polls in California and swing California state elections in the Democrats’ favor.

Ballot measures don’t usually have a large effect on turnout. But if they drive key groups to to the polls they can make a difference. That’s crucial because while polls show just a small advantage for Republicans this year among registered voters, they show a much larger advantage for Republicans among likely voters. In other words, the reason the Republicans are poised to make substantial gains across the country this year is that Republican voters are likely to turn out in record numbers, while many Democratic voters stay home. Proposition 19 might narrow that “enthusiasm gap” in California by getting a more Democrats out to the polls. That could make the difference for Boxer and Brown, even though they have both come out against Proposition 19.

It could also make a difference in 2012. If Proposition 19 gets more Democrats to turn out this year, the Democrats will consider introducing similar measures in battleground states like Colorado, Washington, and Nevada. Blair Butterworth, a Democratic consultant, told Wallsten that legalization measures could boost turnout among the young by 2-4%. “It’s not like a home run,” Butterworth said. “But with elections being so close these days, it’s a big difference.”