Politics does seem to play some part in who gets elected to the Senior Council, even if age is the primary factor. For instance, the Merlin seemed to know going into the meeting in Summer Knight how the election to the SC seat would go, and he made a play to get one of his favorites into the position. The only serious opposition was Ebenezar, and Eb's greater seniority gave him the edge.
And the selection process ended with a general vote of all wizards in attendance, so presumably it's possible for some kind of majority to vote against a nominee.