By Josh Goodman, CQ Guest Columnist

If you want to measure Democrats’ motivation problem in Virginia, it’s helpful to think about a little-discussed category in polling: the unlikely voter.

Polls, of course, sometimes measure the opinions of all adults, sometimes only measure responses of registered voters and sometimes focus exclusively on those who the pollster determines are likely to vote.

When a pollster measures likely voters, some registered voters don’t make the cut (because they say they don’t plan on voting, because they haven’t voted in recent elections, etc.) Those are unlikely voters, though I’ve never heard a pollster call them that. In fact, you’ll almost never hear pollsters talk about this group of registered voters, which is a shame because looking at unlikely voters is a good way to determine who isn’t motivated to turn out in an election.

The Washington Post polled the Virginia governor’s race recently and found what other pollsters have been finding. Republican Bob McDonnell has a big lead over Democrat Creigh Deeds.

Helpfully, the Post released polling numbers among both likely voters and registered voters. McDonnell leads 54 percent to 39 percent among the 601 likely voters who were sampled. His lead is more modest, 47 percent to 40 percent, among all 868 registered voters the Post sampled.

Based on those numbers, it takes just a little bit of math to extrapolate the views of the 267 unlikely voters in the Post poll. The results, based on my calculations, are 42 percent for Deeds, 31 percent for McDonnell and 27 percent undecided. So Deeds enjoys a double-digit lead among those unlikely voters.

That to me is the strongest indication yet that Virginia Democrats are unmotivated this year. It’s certainly not the only one. Recent polls from SurveyUSA and Public Policy Polling showed a likely electorate with more former John McCain voters than Barack Obama voters. Republicans plan to vote, Democrats don’t. After their victories in the 2005 governor’s race, 2006 and 2008 U.S. Senate races and 2008 presidential race, Democrats in Virginia appear to be resting on their laurels.

There’s no good way to lose an election, but if a candidate fails to win swing voters, that at least doesn’t tend to have much of an effect on the rest of the party. Failing to motivate your party to even show up risks a rout up and down the ballot, which is the danger Virginia Democrats face in an important election year.

Control of the state House of Delegates is up for grabs, with major implications for redistricting. Democrats were very optimistic about winning the attorney’s general race and somewhat optimistic about the lieutenant governor’s race until a couple of months ago. Public Policy Polling and SurveyUSA both showed the GOP sweeping the statewide offices, a reflection of the year’s unusually Republican likely voter pool.

Deeds’ deficit among registered voters in the Post poll is evidence he needs to persuade more Virginians that he will make the best governor. Still, his biggest challenge (and the one most relevant to the rest of the Democratic ticket) is persuading people who are predisposed to think he will make the best governor to actually vote. Getting unlikely voters to the polls isn’t impossible, but it is, well, unlikely.

Josh Goodman is a staff writer for Governing Magazine and writes a blog about politics on Ballot Box.