Instapundit, a law professor, demolishes the argument, made by Jerry Pournelle, that Congress should police itself and that the executive branch should not intervene. He writes:

Now you may think that this is a good idea -- I don't, but Pournelle apparently does -- but it is not now, nor has it ever been, the law. In fact, with the sole exception of impeachment (which doesn't run against members of Congress), the Congress cannot investigate or try offenses, and impeachment is carefully distinguished from criminal prosecution in the Constitution. The Constitution's prohibition of Bills of Attainder, in fact, explicitly forbids Congress dealing with criminal matters.

Read it here.

Prof. Reynolds also links to a WSJ article by Bob Turner, UVa Law Prof., on why the constitutional argument being raised by members of Congress is bogus, and why they are so anxious to be immunized from prosecution. It's behind a subscription wall, but Reynolds provides a long excerpt.