But a filibuster would mean we'd need all 55 Republicans and 5 Democrats to break it. Unless we went nuclear, and -- while I think Frist probably (not perhaps definitely) had just enough votes for the rules change -- I don't think we have the votes now. I think Frist had probably been able to dragoon a couple back before the gang of 15, but there were numerous Republicans who were not enthusiastic about the prospect.
And the longer this drags on (Dems have been known to filibuster for years), the longer O'Connor remains on the court. And while they might get pressure to fill a truly vacant seat, there wouldn't be any to fill a seat that's already filled with someone they like better.
Seven Democrats promised very publicly that they would not filibuster except under "extraordinary circumstances". Being a qualified, scandal-free conservative judge does not count as "extraordinary circumstances".
So no, the Democrats could not sustain a filibuster.
That was part of the genius of the "Gang of Fourteen" deal.