If she she and/or The Prez withdraws her nomination; who might his next choice be?
That answer your question?
If Miers does withdraw or bombs at the hearings, my guess is that Bush will ask O'Connor to stay on for the remainder of the term, and start over with a new pick next summer.
The answer that typically comes to that question is "somebody worse." The most often mentioned by name is Alberto Gonzales.
One reason given for it being "somebody worse" is that the President will be angry with the opposition to Ms. Mires nomination, and will strike out to get even. I.e., spite.
Who knows what he'll do.