Just look at the Federalist Papers, #76, in which Alexander Hamilton discusses why the founding fathers gave the U.S. Senate the power to confirm - or reject - the President's Supreme Court nominees:

"To what purpose then require the co-operation of the Senate? I answer, that the necessity of their concurrence would have a powerful, though, in general, a silent operation. It would be an excellent check upon a spirit of favoritism in the President, and would tend greatly to prevent the appointment of unfit characters from State prejudice, from family connection, from personal attachment..."

The passage gets even more specific about political cronies like Miers:

"[The President] would be both ashamed and afraid to bring forward, for the most distinguished or lucrative stations, candidates who had no other merit than that of coming from the same State to which he particularly belonged, or of being in some way or other personally allied to him, or of possessing the necessary insignificance and pliancy to render them the obsequious instruments of his pleasure."