He can't cave at all. Estrada is the test case. If his appointment is shot down, Bush won't be able to get anybody through Senate confirmation. The democrats will be in complete control. Frist and the Republicans either win this one or lose the whole enchilada.
I believe strongly in self determination. If the Democrats are not willing to live under the US Constitution, then we need to live in separate countries. There is no reason for the United States to continue to exist if the Constitution is not going to form the foundation and frame work for the rule of law. And if the Democrats are determined that the rest of us are going to live under the laws rather than within the framework of the US Constitution, then there may come a time when it will be time to get out the guns to decide where we are each going to live and under what laws.
Estrada's nomination and the principles of why he should be confirmed and why Democrat's are determined to block it is a good time and place to begin the debate. Perhaps a duel between Frist and Daschle would be a good starting point if the debate ends in impasse.