Definition 1 of 1
Elder, the singular possibility of the expression is not in use.
Elders are supposed to be part of the checks and balance mechanism of our democracy. Usually indirectly elected or selected, elders are there to take care of certain bulk-constituents' interests. The easily recognizable bulk-constituents could be states themselves at the center, graduates or teachers at the state legislatures, Anglo-Indians, other minorities, Artists, Industry stalwarts and what not.
Seniors are often men and women who fail at the hustings but have enough clout with high commands of parties or are important to the parties that they get accommodated in upper houses by some or the other back door entry tactics.
Powerful politicians who are very inconvenient and at the same time inevitable evils to high commands are often made Governors of states.
Those wo are not clearly 'state-governor-material' but who are still capable of causing big trouble become seniors.
Also, life long faithful servants of the first family stand the best chance of becoming elders.