Illustration by Andy Rementer
Illustration by Andy Rementer

Grassroots political movements such as the Tea Party are a mainstay of American society. But do they really make a difference?

According to one view, the Tea Party is a powerful new force reshaping our politics. The alternative view is that it’s all signs and fury, a bunch of like-minded individuals who gather to cheer on their favorite political causes, but whose activities have no more effect on the outcome than sports fans getting together to watch their favorite team.

What makes it hard to assess these competing views is that the Tea Party is strongest in highly conservative, born-again, Republican communities. So how do we know whether the movement actually influences these people, or merely reflects their changing views and behavior?

If medical researchers were trying to sort this out, they might randomly infect some areas of the country with Tea Party mania, and leave others untouched. By comparing the political evolution of these treatment and control groups, they might hope to learn about the true effects of the Tea Party.

Fortunately, nature sometimes meddles much as our hypothetical experimenters would.

The Tea Party came into prominence in a series of protests around the country on tax day, April 15, 2009. Sunny skies in some parts of the country encouraged large and boisterous rallies, while in other places rain suppressed the attendance. If the areas that nature randomly selected to have good weather that day subsequently became more conservative, that would suggest the Tea Party had a real impact beyond what would have happened in its absence.

Striking Research

How much of a difference can a rainstorm make? It turns out a lot. At least that’s the message from some striking research by four young scholars spanning the political spectrum -- Andreas Madestam of Bocconi University and Daniel Shoag, Stan Veuger and David Yanagizawa-Drott of Harvard University.

Their research demonstrates that in politics, success begets success. The initial boost from the weather generated substantial momentum. Counties that enjoyed better weather on tax day had more people sign up to become Tea Party organizers, greater donations to an affiliated political action committee, and larger rallies a year later.

It’s easy to imagine how this works. Showing up at a rally increases the chances of getting more involved, making a donation or bringing a friend to another event. Larger and more successful rallies also boost subsequent news media coverage of the movement, further stimulating community interest.

What’s more, the Tea Party experiment shows that the activism catalyzed by those sunny days translates into real political influence. Politicians whose districts were sunny on tax day voted in a more reliably conservative fashion throughout 2009 and 2010. Indeed, the absence of rain in a congressional district on April 15, 2009, made its representative 8.7 percentage points more likely to vote against the Affordable Care Act. Had the weather at those early rallies been sunnier, it’s possible that Obama’s signature legislation wouldn’t have passed.

The Tea Party also changed the makeup of Congress. Sunny days -- and thus large rallies in 2009 -- led to greater Republican turnout and slightly weaker Democratic participation in the 2010 midterm elections. This boosted the Republican vote share by almost three percentage points in the affected districts. There was even some impact before Election Day: Several Democratic incumbents, seeing the writing on the wall, decided not to seek re-election.

The Next Election

It’s rather striking to realize that a sunny day can have such big repercussions for our political landscape.

One can’t help but wonder whether the Occupy Wall Street movement has had similarly far-reaching effects. The researchers behind the Tea Party study tell us that weather-driven momentum was also important for Occupy protesters. We won’t know the broader effects of this movement until we see the 2012 election results.

Already, though, we are seeing some suggestive evidence of the Tea Party’s influence on the 2012 elections. Mitt Romney, who has less support from the movement than his opponents, eked out close wins in the Ohio and Michigan primaries -- states where rain on that fateful day in 2009 muted the strength of the Tea Party.

As attention now shifts to the Illinois primary, we suggest that pundits consider one extra fact: In 2009, tax day was mostly dry in Illinois.

(Betsey Stevenson and Justin Wolfers, both professors at the University of Pennsylvania’s Wharton School, are Bloomberg View columnists. The opinions expressed are their own.)

Read more opinion online from Bloomberg View.

To contact the writers of this article: Justin Wolfers at jwolfers@wharton.upenn.edu or Betsey Stevenson at betseys@wharton.upenn.edu.

To contact the editor responsible for this article: Mark Whitehouse at mwhitehouse1@bloomberg.net.