In the judge context, as it has been played out in the Senate, the mainstream should be defined as the whatever 51 Senators say it is, but the inverse is true as well. If a nominee is so out of the "mainstream" as I or anyone as defined it, if that nominee is so "extreme" in their legal philosophy, then Senator of both parties should recognize that and vote down the nominee.
Since that hasn't happened, it is telling, that the objection to these nominees isn't based completely on their philosophies, though that may be partly true, but the objection to these nominees is based on politics. Democrats don't think that President Bush ought to be able to appoint federal judges because he IS George W. Bush and was "selected, not elected" and despite his RE-election, the people who voted for him are right-wing Christian rubes from Jesusland, and THOSE people should not have a say in who gets nominated to the judiciary and THOSE people's political desires ought to be thwarted because THOSE people don't count to the liberal elitests who have had their personal policy preferences dictated by unelected judges for 70 years.
Actually, when one considers the view of the judiciary that the lefties have, their reaction is appropos. If the judiciary is the place where policy preferences are enshrined into untouchable LAW and removed from the democratic process, then of course, lifetime appointments to these positions ought to require 60 votes. The left fears that conservatives will have their policy preferences enshrined and untouchable for the next 70 years and there will no recourse through the democratic process to undo what the courts have done.
The Dems filibuster arguments don't make sense logically, but their reaction makes sense because they view the courts as the great ruling judiciarchy.