There is no constitutional power that allows congress to really, absolutely force a sitting president to do anything. They do however determine funding, and they can impeach(which isn't on the table), which gives them a VERY big stick.
So if the Miers/Justice dept scandal is really that big a deal, use the fucking stick. Start defunding things.
The executive has, imo, usurped wayyyy too much power over the past 50 years. Bush has followed in the footsteps of his predeccesors and finally, really pushed it over the line. So congress needs to grow a pair and rein the branch back in, or shut the fuck up about it.
All of congress's oversight is basically based on the premise that if they don't get to do it, they won't fund it. Because while it's a tradition, it's not a power.