There are, of course, a few problems here. The first one is that, when you start borrowing talking points — the president wasn't "vetted" in 2008 — from Princess Dumbass of the Northwoods, you're already running a few lengths below the intellectual Mendoza line. The second is logical; we already know far more about the president's relationship with both Tony Rezko and cocaine than we do about Willard Romney's relationship to the U.S. tax code for the years, say, 1999-2008. The third is strategic; if you have to remind people that you're preparing to get tough, you've pulled your own punch before throwing it. And the last one is perceptual; Willard Romney — and most of his surrogates — look utterly ridiculous in the role of political hatchetmen.
This is the new, tougher Willard Romney, who is so damned rugged that he's afraid of what the president's people might do with the information in his tax returns. This is the saloon brawler who won't shut up about all the mean things the president's "opposition research" might do to him. This is Willard Freaking Romney, for pity's sake, of whom an "adviser" warns the president:

Mr. Romney says he cannot be held responsible for a Bain-owned steel plant that went bankrupt in 2001, claiming he had no active operating role at Bain Capital after 1999. But the Boston Globe, citing corporate documents filed by Bain with the Securities and Exchange Commission, reported that Mr. Romney remained chief executive of the company through 2002.