If you’ve gotten the impression that the Republican presidential nominating contest has become a wildly negative affair dominated by billionaire-financed Super-PACs, well, you’re not just having a psychic flash. As an alarming new report by WaPo’s T.W. Farnam indicates, something rather unprecedented is going on:

Four years ago, just 6 percent of campaign advertising in the GOP primaries amounted to attacks on other Republicans; in this election, that figure has shot up to more than 50 percent, according to an analysis of advertising trends.

And the negative ads are not just more frequent — they also appear to be more vitriolic.

Pretty much everyone looking at the descent of the contest into viciousness points the finger at Mitt Romney and his Restore Our Future Super-PAC for leading the way. It was Team Mitt that first went heavily negative in Iowa, destroying initial leads by Rick Perry and then Newt Gingrich. The numbers support that narrative:

Romney and the groups backing him have led the trend, spending two-thirds of their money on negative ads. Gingrich and the Winning Our Future PAC backing him have spent half of their funds on spots attacking other Republicans. Santorum and the PACs behind him have devoted one out of four dollars to attack ads….

“Mitt Romney has resorted to a carpet-bombing strategy that helped him win some early primaries,” said Mark McKinnon, a political strategist for McCain and George W. Bush, “but his favorable impression among independents has collapsed. Seems likely there is some correlation.”

Winning Our Future spokesman Rick Tyler said his group’s message was positive until it was forced to counter Romney’s “scorched earth” strategy.

Yeah, well, Winning Our Future sure caught up in South Carolina, with ads and even an entire movie that painted Mitt Romney as a quintessentially evil man who personally swooped into the state to take away jobs for the sheer pleasure of watching people suffer.

But no one much wants to take responsibility for this dumpster-dive of a campaign season. In an interview with Forbes, Winning Our Future’s principal donor, casino mogul Sheldon Adelson, makes this astonishing claim:

“I don’t believe in negative campaigning. I believe in saying that my opponents are very good people and I’m confident a lot of them would do a good job, but I would do a better job, and here’s why,” says Adelson. “Money is fungible, but you can’t take my money out of the total money you have and use it for negative campaigning.” Of course, that stance ignores the fact that an avalanche of negative ads against Romney won Gingrich South Carolina, and that Adelson’s $5 million injection was the dominant source of his funding. “That’s what everybody says, but that doesn’t mean it’s true,” the billionaire says, waving his hands dismissively. “Most of what’s been written about me in this is untrue.”

That’s the spirit of 2012, Sheldon! Just brazen it out!

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.