Meaning of two-thirds rule

two'-thirds rule'

Pronunciation: (t'thûrdz"), [key]
  1. a former rule in the Democratic party, effective 1832–1936, requiring a vote of at least two thirds of its national convention delegates to nominate a presidential and vice-presidential candidate.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.