A small but definite chunk of Americans don't know much about Mitt Romney.

Romney's blank-slate quality was highlighted by the latest swing-state poll, taken by Quinnipiac in Virginia and released Thursday: When asked for their opinions on Romney, a full 21 percent said they hadn't heard enough about him to decide.

The same was true last month in Florida and Ohio, where Quinnipiac polls showed 19 and 24 percent, respectively, had no opinion of the man who could move into the White House in January. Even more independents said they had no view-24 percent and 28 percent, respectively.

The average person wouldn't be disappointed if one in five swing-state voters held no opinion of him or her, and perhaps Romney shouldn't take it so hard, either. But the polls indicate a difference between Romney and Obama, one that should perhaps be obvious: Obama, having been president for three and a half years, is a known quantity-in each of those states, all but four percent or fewer had an opinion of him; Romney, less so.

It's not that people don't like Romney. He was popular in Virginia (39 percent favorable, 37 percent unfavorable) and Florida (44 percent favorable, 35 percent unfavorable); in Ohio, he was narrowly unpopular (35 percent favorable, 37 percent unfavorable). It's just that they haven't made up their minds.

We shouldn't read too much into the findings of a single polling agency. Each asks questions differently, and some push respondents to offer an opinion, while some don't. Quinnipiac doesn't, meaning higher responses of no opinion. Other pollsters have found more affinity and aversion, where Romney is concerned.

The latest ABC News/Washington Post polling, for instance, found that 14 percent hold no opinion of Romney nationwide. (He's unpopular, according to ABC's findings, at 41 percent favorable, 45 percent unfavorable.) But again, Obama is a known entity, with only three percent expressing no opinion of him.

The public feels less strongly about Romney, too. In the May 23-27 ABC/Washington Post poll, 60 percent of respondents had strong feelings about Obama, one way or another, far more than the 47 percent who were less sure; when asked about Romney, 39 percent had strong feelings, fewer than the 47 percent who felt "somewhat" positively or negatively toward him.

Whether these numbers are higher or lower than normal is debatable-Quinnipiac found 15-percent non-opinion on John McCain in July 2008-but it's less important than what they mean for the campaign. Double-digit uncertainty means that, while opinion on Obama may or may not change, there is a more definite opportunity to shape what Americans think of Romney. Which is perhaps why, after running an almost-exclusively positive race in 2008, Obama's team has hit Romney on his career at Bain Capital and as governor of Massachusetts so early in the 2012 campaign.