Compared with Presidential election years, young voters turnout in much lower numbers for midterms. This is a concern for supporters of Proposition 19, a ballot measure in California seeking to legalize, regulate and tax marijuana. Young adults are the demographic most likely to vote “yes” on Prop 19. But if Prop 19 serves as a motivator to get young adults to the polls in California this November, it could significantly change the makeup of the electorate, possibly helping Prop 19 pass.

To give you an idea of the drop-off in youth turnout between presidential and midterm elections, here is the age breakdown of turnout in California for 2006 and 2008 (my thanks to the California Sectary of State’s Election Division for the information):

In the last midterm election, voters age 33 or younger made up only 13.6 percent of the electorate, yet in the 2008 presidential contest, the same group made up over 23 percent of the vote. That is a substantial increase in the relative number of young people voting—a level of youth turnout that could mean the difference between the success or failure of Prop 19.

Let’s assume having Prop 19 on the ballot does motivate young people to turn out so they make up relatively the same amount of the electorate in 2010 as they did in 2008. That could result in roughly a 1 to 2.5 percent increase for “yes on 19” and a corresponding decrease for “no,” depending on what poll crosstabs you use (I looked at both California Prop 19 Field (PDF) and SurveyUSA polls and national polls for additional comparison). If young voters make up as big a share of the vote in 2010 as they did in 2008, that alone could make the difference between Prop 19 losing narrowly (say, 49%-51%) or winning by a similar margin. The impact could be even greater if the bulk of the theoretical increase in youth voters comes mainly from enthusiastic young Prop 19 supporters, and you assume young anti-19 voters might feel more ambivalent about marijuana legalization.

Just as a thought experiment, let’s assume Prop 19 makes 18-33 year-olds as excited to vote in 2010 as they were in ‘08’s presidential race between Barack Obama and John McCain, and as many young people vote this November as they did in 2008. But for all other age groups, the turnout remains at more traditional midterm levels. If that happened, 18-33 year-olds would make up roughly 32% of the electorate. Under this scenario, massive youth turnout could result in roughly a 2 to 4.5 percent upswing for “yes on Prop 19.” (I fully acknowledge the improbability of this scenario, and the difficulty in convincing all the people who voted in 2008 to vote again this year. This is just meant to show the upper limits of what very large youth turnout could theoretically do to help Prop 19.)

If the vote on Proposition 19 ends up as close as most people expect, a large increase in turnout from motivated young supporters could make up that few percentage points that push it over the top. The question is: can marijuana legalization motivate a decent fraction of the many normally politically disengaged young adults to vote at the levels we saw when Obama and McCain were on the ballot? I think there are some signs that potentially point to yes, but we will find out this November.