The fight for the millennial voter; change happens on the battlefield of ideas, not as the result of elections.

• Author: Rugy, Veronique de
• Position: Columns

For many libertarians, it's a recurring frustration: Why do candidates who seem attractive before the race begins suddenly sound so much squishier once they hit the campaign trail?

Take Rand Paul, the Kentucky senator with high hopes of becoming the Republican presidential nominee in 2016. He's far more libertarian on foreign policy and the surveillance state than most members of his party. But in recent weeks, he has taken a harder-line stance on using military force, even calling for an increase in overall military spending. Likewise, Rep. Paul Ryan (R-Wis.), 2012's closest thing to a Republican champion of entitlement reform, suddenly reversed course and delivered a passionate speech in defense of Medicare spending at the Republican National Convention once the vice presidency was on the line.

A concept from economics called the median voter theorem provides one explanation for this wobbliness. In a video for Learn Liberty, the Creighton University economist Diana Thomas explains that our majority-rule system means candidates are forced to position themselves strategically to try to win more than 5o percent of the vote. Since a Democrat in a two-party race knows she can more or less count on the support of everyone to her left, for example, it behooves her to put as many people as possible to the left of her.

"Even if a candidate starts out on an extreme end of the political spectrum, he ultimately will aim for the middle [because he needs] to convince the 'median voter' to vote for him," she says. That is why politicians try so hard to reflect the preferences of the "average" American.

The same-sex marriage issue provides a great case study of this phenomenon. Until recently, an overwhelming majority of voters thought marriage ought to be reserved for heterosexual couples only. As long as the public took that position, most politicians --on both sides of the aisle--were happy to follow suit. But the last decade saw that trend reverse. Support for marriage equality has remained above 50 percent in most polls since 2010.

As public opinion changed, so did the public stances taken by many politicians. President Barack Obama, who insisted in 2008 that marriage should be between one man and one woman, did an about-face in 2012. Hillary Clinton has also "evolved" on the issue since deciding to run for president again in 2016. Even some Republicans have come out in favor of gay marriage.

Libertarians' goal, then, should be to make the median...