Oversight
The idea that this process had no oversight is false. It was run past the ranking members of the intelligence committee, and others, to no real objection.
In short, the idea that the executive branch did this all on its own is a lie.
This is why many Democrats aren't really hoping for a real investigation. If they can run on bluff and innuendo, hoping nobody actually looks at what really happened, they can accomplish their political goals. If the details are actually looked at, more than a few Democrats are not going to be looking so good. Rockefeller is still sweating bullets.