TWO-THIRDS RULE. The two-thirds rule is used at all levels of government and in many social and political organizations to prevent the dominance of a small majority over a large minority. The U.S. Constitution, for example, gives the Senate sole authority to ratify treaties proposed by the President of the United States and to try impeachments but makes this contingent upon a two thirds majority, thus ensuring broad support for such important measures. In 1832, the Democratic Party adopted a two-thirds rule for nominating a presidential candidate. Frequent attempts to change the rule were resisted by those who believed it to be a convenient tool to prevent a candidacy they opposed. It was finally repealed in 1936.
Bass, Harold F. "Presidential Party Leadership and Party Reform: Franklin D. Roosevelt and the Abrogation of the Two-Thirds Rule." Presidential Studies Quarterly 18 (1988): 303–317.