As Virginia prepares to elect its next U.S. senator, how will the state's millennials cast their ballots? A statewide poll released today finds young people prefer "anyone but Republican Ed Gillespie." Bloomberg Politics reports:

Democratic Senator Mark Warner captured 47 percent in a survey of voters between the ages of 18 and 35, which was released Thursday by the Wason Center for Public Policy at Christopher Newport University in Newport News, Va. The first runner-up was Libertarian Robert Sarvis with 24 percent. Eighteen percent said they were undecided, and 11 percent said they will choose Gillespie. [emphasis added]

In other words, the libertarian candidate appears to enjoy six times as much support from the millennial generation as from the electorate at large. (Real Clear Politics' polling average currently puts Sarvis at just 4 percent.) However, there are some very good reasons to think that level of support for Sarvis from young voters might not actually materialize.

In a previous post here at Hit & Run, I discussed the phenomenon of polls tending to overstate third party candidates' preformance on election day. But there's another problem with taking this finding at face value, which the Bloomberg article itself points out: It assumes young voters will show up at the polls.

It's unclear how many millennials will actually go to the polls. "A majority say they are certain to vote, but only 44% say they are paying close or somewhat close attention, so it's easy to imagine many who might intend to vote not actually making it to the polls on Election Day," said Wason Center Director Quentin Kidd in a news release.

Historically, young people cast ballots at far lower rates than older voters. According to a study from the U.S. Census Bureau, released in April:

In every presidential election since 1964, young voters between the ages of 18 through 24 have consistently voted at lower rates than all other age groups…Overall, America's youngest voters have moved towards less engagement over time, as 18- through 24-year-olds' voting rates dropped from 50.9 percent in 1964 to 38.0 percent in 2012.

But that's not all—even older voters have a track record of being, shall we say, overly ambitious when reporting their likelihood of voting. Consider the Scottish independence referendum as one high-profile example from this year. An Ipsos MORI poll taken just before the election found some 95 percent saying they were certain to turn out—a 10 on a 10-point scale. In fact, just under 85 percent of registered voters actually cast ballots—a "record number," and no wonder considering the historic nature of the election. But it still wasn't 95 percent.

And the number of people who misrepresent their vote likelihood is often much larger than that. A 2013 Harvard Kennedy School study looked at a series of races and found that in all of them, "a sizable fraction of those who self-predicted that they would vote mispredicted and did not actually vote." In one case, more than half of self-predicted voters failed to turn out.

So while a majority of Virginia millennials might believe themselves to be certain to vote—and nearly a quarter say they'd vote for Sarvis—chances are, quite a few of them are mistaken.