The power of the press.

Chin, in the Editor's choice section of Science, notes work from Gerber et al. on the effects of newspapers:

A common belief in the United States is that the media exhibit a liberal bias, which generally aligns them with Democratic programs and politicians, in their reporting of the news and in their selection of what news to report on. In fact, one study estimates the effect of Fox News Channel, which was launched about a decade ago and is generally more conservative than other television outlets, as having increased the Republican share of the vote by half a percentage point.

One month before the November 2005 gubernatorial election in Virginia, Gerber et al. carried out a randomized field study in which several thousand households that did not already receive a daily newspaper were given trial subscriptions to either the Washington Post (liberal) or the Washington Times (conservative). Post-election telephone interviews established that receiving either newspaper had little impact on factual knowledge (such as Harriet Miers being a Supreme Court nominee) or political attitudes (such as President Bush's approval rating). What was affected was voter turnout (as measured by administrative records) and, surprisingly, actual voter choice, with both sets of newspaper-receiving households favoring the Democratic candidate by about seven percentage points.