My belief is that in this political climate with a weak Senate leader and squishy Republican Senators, the President knew ahead of time that anyone like Luttig or Jones with a long paper trail of Conservative decisions would never even muster 51 votes.
For the sake of argument, let's say that one of the preferable candidates for this position weren't able to muster a sufficient number of votes to quash a threatened filibuster-a filibuster that is a matter of supposition-it would not be the end of the world.
President Bush could have then nominated Harriet Miers, or any of a number of less qualified individuals, as a replacement, as was the case when Judge Bork's nomination was rejected by a majority of dimwitted, politically vindictive Senators, who did not have even the most rudimentary grasp of Constitutional principles.
That is a reasonable belief, and perhaps the most powerful argument in support of the nomination. But again, it is speculation, and contradictory. W is saying (in effect) that Miers will be a Thomas or Scalia. As such, why are we to believe she is more acceptable to squishy senators, when she will be a Thomas or Scalia without any evidence of her ability, unlike Luttig or Brown?