It’s not just about whether Romney did something illegal, and I kind of doubt he did although I’m not ruling it out.
Another big problem for him is Bain’s investment in Stericycle, the company that disposed of aborted fetuses. The religious right is already freaking out about it, and if it’s shown that Romney was still involved with Bain at that point, it could seriously hurt him in the anti-choice movement.
He’s spent years pandering, bowing, and scraping to the fundamentalists despite being a Mormon, and has a strong motivation to obfuscate anything that might damage this fragile truce he’s built with them.