So, now we know.

After months of his campaign dodging direct questions on the matter, Mitt Romney said Thursday in South Carolina that at no time in the past decade has he paid less than a 13 percent tax rate.

"I’ve paid at least 13 percent and if you add in addition the amount that goes to charity, why the number gets well above 20 percent," Romney said at a press availability in Greer, South Carolina. (The Romney team notes that this is only the second time -- the first was at a press availability in Europe -- that the candidate has been directly asked about his effective tax rate.)

Here's Romney's full answer from today:

Romney's answer won't satisfy everyone. (More on that below.) But, in asserting that for the last decade he has never paid less than a 13 percent tax rate, Romney is calling Democrats' bluff and forcing them now to call him a liar if they argue that he paid any less. In short: The burden of proof has now shifted from Romney to Senate Majority Leader Harry Reid and the Democratic party more broadly.

Remember that it was Reid who drew national -- and international -- headlines for his repeated assertion that a "source" had told him that Romney had paid no taxes at all for the better part of a decade. (Reid's actual quote: "The word is out he hasn’t paid taxes for 10 years.") Reid has both refused to reveal the source or to back down in any meaningful way from his allegation.

"Harry Reid’s charge is totally false," Romney said in South Carolina today. "I’m sure waiting for Harry to put up who it was that told him what he says they told him. I don’t believe it for a minute, by the way."

Reid's response? "We'll believe it when we see it," said a spokesman for the Senate Majority Leader.

Let's be clear: For Democratic partisans, Romney's answer won't be enough.  They will insist he is either obfuscating or outright lying and that the only way to for him to prove that he is being truthful is to release his full returns. And, as we wrote this morning, that almost certainly won't happen.

"Given Mitt Romney's secrecy about his returns, coupled with the revelations in just the one return we have seen to date and the inconsistencies between this one return and his other financial disclosures, he has forfeited the right to have us take him just at his word," said Obama campaign spokeswoman Lis Smith in a statement released Thursday afternoon.

But with Romney's on-the-record insistence regarding his tax rate, Obama, Reid and other Democrats will have to, well, put up or shut up in order to keep the "no taxes" story alive.  They will have to say that there is evidence that in some year, Romney paid no taxes --using that approach to attempt to force his hand on releasing more detailed information. Otherwise, they are asking the American public to believe that a major party presidential candidate willingly lied about his financial past. And that's a major leap.

What smart Democrats should do is not fight the fight over whether Romney is lying about his tax rate but rather continue to push for the former Massachusetts governor to release a more detailed accounting of his financial past.

That seems to be the official White House approach to the issue.  "This president believes the tradition for presidential candidates putting forward multiple years of tax returns is a useful and valuable one," said White House press secretary Jay Carney. "It's not always a comfortable one but it's one he abided by and one the American people believe is right and expect their candidates to abide by."

We've written before that it's a lock that Romney's tax returns will show a financial life that is exotic in the extreme as compared to the average person he needs to vote for him this fall.

Taking that approach -- rather than fighting Romney over what rate he paid --  allows Democrats to keep the issue alive, which is a very good thing for them in the context of the campaign.

Romney's gambit on taxes today is clearly aimed at taking the tax return issue of the table. It almost certainly won't do that but he has changed the game when it comes to the debate.