Rational?

The Kennedy/Kerry filibuster is the tactical best play for the Dems. It will appease the liberal base - the Dem senators "fought." It will give several Dem Red State senators the opportunity to split their votes: no to Alito but also no to filibuster. And it will certainly fail, so will not bring on the nuke option, thus preserving the filibuster to fight another day.

This false filibuster also increases the probability of Chaffee voting against Alito...