Clinton - Lewinski led to impeachment.
Bush, Cheney, Rumsfeld led us to Iraq war. Evidence was fabricated and when called out, Ms. Plame was outed. So there is a huge credibility gap.
And now Boehner and his proposed lawsuit? Pure dysfunction and I believe that the moderates and the left are getting sick and tired of the political drama.
As to IRS, if something is there, it is bound to come out. Wouldn't you expect the Republicans in IRS to be leaking in droves by now if there is criminal wrongdoing?
EDIT: Especially considering the diversity in the GOP.