My assertion is that no evidence has been produced linking the White House to the IRS's targeting of conservative groups.
Well, that's a silly, partisan assertion.
Now you can question the importance of such evidence as has surfaced and is known. You can question veracity of sources, or quality of that evidence. You can discount it. But there certainly is evidence, even at this very very early stage of discovery. Saying there is "no evidence" is simply not true.
E.g., Obama inexplicably met with the president of the Anti-Tea Party National Treasury Employees Union, Colleen Kelley, at the White House the day before her IRS Union Employees agreed to target the Tea Party.
What was discussed at that meeting?
As this evidence is further ferreted out it may or may not be critical, but even now it's evidence, albeit of undetermined importance.