Virtually every political observer following the presidential race put enormous import on the upcoming debates. (“Debates are Romney’s last stand,” reads a typical piece.)

That may seem like a silly way to decide who leads the free world, especially when both candidates have spent months, even years, articulating their positions on matters of national importance. Maybe for some elections, that’s true—but Mitt Romney has assiduously avoided taking stands on various important issues, so the debates present a minefield for the Republican candidate. If the moderators aggressively press him to define a particular policy position on the fly, and he chooses his evasive words poorly, he may suddenly find himself in a huge mess.

Such is the lesson Democrat Tim Kaine presented today in a debate with Republican George Allen in the Virginia Senate race, committing what Dave Weigel called “One of the most obvious unforced errors I’ve ever seen.”

Debate moderator David Gregory tossed what should have been a softball question to Kaine about Romney’s derision towards the 47 percent of Americans who pay no federal income taxes. But it turned out to be a change-up pitch, as Gregory repeatedly pressed Kaine to say whether everyone should pay federal income taxes—and Kaine, shockingly, was open to the idea:

GREGORY: Do you believe that everyone in Virginia should pay something in federal income tax?

KAINE: Well, everyone pays taxes! I mean, the statistics that have come out..

GREGORY: I’m asking about federal income taxes.

KAINE: I would be open to a proposal that would have some minimum tax level for everyone, but I do insist, many of the 47 percent that Governor Romney was going after pay a higher percentage of their income in taxes than he does.

Openness towards taxing that 47 percent is a deeply foolish position to take. (Evidence: the only one I’m aware of who has even come close is Michele Bachmann). As just about every Democratic politician and liberal commentator has spent the week proclaiming, most of that 47 percent are justifiably unable to pay income taxes because they don’t make enough money—that’s exactly why Romney is perceived as cruel for those remarks.

Kaine either didn’t internalize that argument and was just trying to brush Gregory off (likely), or is actually in favor of taxing low-income people (very unlikely, and nowhere in his tax plans).

But now he’s stuck—unwilling to say he screwed up, and to boldy reverse a position forty minutes after taking it, Kaine and his staff instead are highlighting the “open to a proposal” part of the remarks. While pointing out that as governor he never raised taxes on low-income Virginians, and stating it’s not his preferred policy, Kaine told The Hill after the debate that “It shouldn’t be news that somebody wants to go into the Senate that is willing to start with a position of openness and a dialogue.” A staffer told Talking Points Memo

“You can’t issue comprehensive tax reform if you start shutting doors.” (Sure, but you can rule out damaging, punitive policy choices. Does Kaine leave the door open on taxing all of the state’s redheads?)

This wiggling won’t be enough. Immediately after the debate, Allen—who cannily never brought Kaine’s remarks back up during the debate, thus denying Kaine a chance to reverse himself more smoothly—stuck the knife in. “It’s typical of Tim Kaine. His record is always one looking to raise taxes,” Allen said. Within the hour, his campaign was blasting out an e-mail titled “Tim Kaine Says He Wants ‘Minimum Tax Level for Everyone.’” I would guess that television advertisements will be up by Monday.

This was the week Republicans were supposed come off as cruel to the “47 percent,” but Kaine managed to implicate himself as well because he wasn’t prepared for a debate and couldn’t offer policy specifics.

Given the vast number of policy areas in which Mitt Romney simply has no specifics—and given his proclivity towards damaging statements during live debates—the Republican nominee ought to take note. Meanwhile, Tim Kaine may have just blunted whatever polling momentum was headed his way.