IMHO, considering current Senate realities, Dubya figured he had a better chance winning a fight with his base over a known quantity (that the base will like), vs losing a fight over a publicly known quantity.
I find it hard to believe GWB did not anticipate the firestorm that would be coming from the base on this. He had to choose between fighting his base or fighting the moderates and Dimwits in the Senate, and he made the political calculation that we were the less formidable foe.
He knew the Dims were going to like the pick. Reid recommended her. Miers has a record of being someone the Dims can work with. Her law firm is a significant contributor to their campaigns, and she was quite willing to go along with most Dimwitted initiatives (with abortion being the one exception) when she was on the Dallas City Council. They see her as someone who is politically pragmatic and malleable (translation: Sandra Day O'Connor).
We aren't out her in the hitherlands singing "I'm just wild about Harrie...."
Well, someone else pointed out that, politically, he needs Harry Reid and the Dims more than he needs his base if he wants to move forward with any kind of agenda. Given what I've seen of his domestic agenda so far, I shudder to think what might be next. Can there possibly be another government boondoggle or pork barrel project we can throw more money at?
I hate to speculate, but I think it could be more. I think he is worried that the administration is going to take a big hit from the special prosecutor later this month, and he thinks he needs more friends among the moderate/liberal majority in congress . Given the growing unpopularity with the war, the fact that the Sunnis could go into open rebellion after the constitution, the yield curve flatlining and a recesion for 2006 looming, the hit he took on Katrina, his tanking poll numbers, etc. the timing was not right to put another difficult item on the agenda. So he took what he calculated was the easier course.
The brouhaha over Miers from the right is nowhere near as harsh and bitter as would be the brouhaha from the left and the MSM had he picked Owens or Brown.