I would like to start with a quote by Kagro X
The failure even to hold Gonzales accountable for his part in this would be a complete abdication of the Congress' responsibility to preserve and exercise an absolutely critical element of the ongoing scheme of accountability designed by the Founders. It was to the Congress that the Founders entrusted the only mechanisms of mid-term accountability that could be brought to bear directly on the president and his administration. Giving that up would be an error of incalculable magnitude.
Impeachment is where the House of Representatives would investigate the Attorney General and determine if he should be charged with impeachable offenses. In terms of normal criminal proceedings it is more akin to a grand jury hearing to determine if charges should be brought.
It is not a step to only be taken when there is clear grounds. It is the step taken when there is a strong reason to believe that the individual has committed impeachable offenses. The House impeachment hearings are to determine if the individual has in fact done so.
After that step, and only if the House votes for impeachment, then the trial occurs in the Senate. And the House would have to find strong evidence of clearly impeachable actions to even bring the charges to the Senate.
The bottom line? The bar for the House to hold impeachment hearings is not that high.
The Grounds
So now comes the question of what grounds do we have. And I think the number 1 reason, of and by itself sufficient to hold these hearings, is not the attorney firings. It is not the illegal wiretapping. It is not the obvious lying to Congress. It is not any of the other political decisions made by the DOJ.
The number 1 ground should be simple fact that Alberto Gonzales is clearly incompetent. He clearly is incapable of running the Department of Justice. And that alone is grounds for impeachment if he will not resign and the President will not remove him.
In addition, this is an argument that most of the country would support. It would be hard for even the most rabid Republican to claim that Gonzales is qualified for his job. On this issue politically impeachment is possible now.
And that's the beauty of this approach. The hard step is starting the hearings. Once they start, all these other impeachable offenses will also come up. But it's the incompetence grounds that can get it started.