Shannon Stapleton / Reuters Republican presidential candidate Mitt Romney is seen in a tele prompter reflection waiting to speak to supporters at the Chillicothe Victory rally in Chillicothe, Ohio, Aug. 14, 2012.

Have you heard the news? Mitt Romney revealed today, after weeks of avoiding the issue, that he has paid at least 13% in taxes in each of the last 10 years. The response from the Obama campaign: “Prove it.”

Just another day of Show-Me-Your-Papers politics. Dana Milbank makes a good point Thursday in the Washington Post: The underhandedness of the current presidential campaign has a lot to do with one side changing its approach. “What’s different this time is that the Democrats are employing the same harsh tactics that have been used against them for so long, with so much success,” Milbank writes.

For years, Barack Obama has been hounded by absurd accusations founded on nothing but weak, often imagined, speculation. Such as: he is not an American. If he were, he would show us his birth certificate. He is not a good student. If he were, he would release his college transcripts. It’s a go-to move for Donald Trump.

It is rooted in a logical fallacy, once reserved for conspiracy theorists, that feels right for many in a partisan age: A claim can be considered valid as long as there is not sufficient proof that it is invalid. Make up anything and it is debatable, unless disproved with documents. I heard Joe Biden spends thousands of dollars a month talking to a pay-per-minute telephone psychic. Absurd, you think? Then why doesn’t he release his phone records? There is a rumor that Paul Ryan has photos of sad cats wearing boots in the top right drawer of his Congressional office desk. A slander? Then why doesn’t Ryan unlock the drawer and let everyone see what he keeps there? (For the record: The psychic and the cat boots are fiction.)

You probably have already heard the latest example of this. Democratic Senate Majority Leader Harry Reid, a soft-spoken political brawler of the first order, said a few weeks back that someone connected with Bain Capital told him once that Mitt Romney had paid no taxes in some recent years. Reid had no proof, and would not reveal his source, or why the source somehow gained access to Romney’s private tax returns. But the rumor was enough.

(PHOTOS: The Rich History of Mitt Romney)

The unfounded accusation in Washington soon become conventional wisdom in some corners of the country. On Monday, at a rally in Boone, Iowa, I found myself talking on the rope line with a delightfully friendly Obama supporter, who volunteered the fact that he was certain that Romney had not paid any taxes, and that was the reason he was not releasing his tax returns. I could not bring myself to write down his name, or his direct quotes, which in retrospect was probably a mistake.

The baseless charge about Romney’s taxes soon made it into questions from reporters, and the Romney campaign initially tried to dismiss it. On Thursday, at the end of a press conference in which Romney had focused on Medicare, he got a question about the Reid charge and said this:

I just have to say, given the challenges that America faces – 23 million people out of work, Iran about to become nuclear, one out of six Americans in poverty – the fascination with taxes I’ve paid I find to be very small-minded compared to the broad issues that we face. But I did go back and look at my taxes and over the past 10 years I never paid less than 13 percent. I think the most recent year is 13.6 or something like that. So I paid taxes every single year. Harry Reid’s charge is totally false. 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. But every year 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.

This is a huge victory for the Obama campaign, and for all those who believe in smarmy politics. A baseless allegation will now cause another round of rumination on Romney’s taxes, which is exactly what the Obama campaign wants. There is no evidence that Romney ever broke the law in his taxes, but there is a lot in Mitt Romney’s taxes that shows how different the tax code works for the very wealthy, which is a message that Obama wants to drive home. Romney’s taxes are also embarrassing for him politically, as they show off-shore holdings, Swiss bank accounts and other investments that do not perfectly match with his policy approach. They have become shorthand for Romney’s financial otherness. The ones that have not been released are now a liberal MacGuffin, in much the way that Obama’s birth certificate and college transcripts are conservative MacGuffins. They are the objects that keep the narrative going.

However unfair and misleading, MacGuffins work in our current national politics. So expect more in the future. Sadly, for all of us.