The democrats don't have to mobilize for this SCOTUS confirmation. There is no doubt that Rehnquist will retire due to bad health.
1. The prevailing wisdom before O'Connor announced ahead of Rehnquist was that the Pres. could nominate an equally conservative judge as Rehnquist and, since it wouldn't tip the balance, that nominee would be given a hard time, but probably be permitted to pass through.
2. If the Dems allow this nominee a pass, instead of Rehnquist, then the above equation hasn't really changed. They simply fight like crazy on the 2nd nominee and not on the first. It's numbers and sequence that matter, not faces.
3. Therefore, it is to the Dems advantage to obstruct and make a scene on the O'Connor nomination, but to accept the eventual appointment of that nomination. If nothing else, it will give them opportunity to test market obstruction ideas and to avoid the nuclear option until a later date when it might be so far on the back burner that it is no longer a useful tool.
4. It is possible, too, that Rehnquist might want to hang on a bit longer to see what impact he could have on a conservative term of SCOTUS. After waiting his whole career for such an opportunity, he might just hold out to be a part of it. He would then announce in the midst of the mid-term elections, and it would give both sides the opportunity for enormous fund-raising on the issue. The advantage for the Dems would be that they get a chance to run a constant fear campaign about the impact of a so-called "right-wing judiciary."
5. But, the dems aren't that bright. They'll go to the wall on O'Connor, but they'll lose and maybe even force the enactment of the nuke option.
You're right, they just can't help themselves.