Two-thirds rule

A two-thirds rule is usually a legal and constitutional requirement that for a proposal to be accepted, it must be supported by at least two-thirds of those voting. In the United States government, a presidential veto of a congressional bill can only be overturned by a two-thirds vote of the U.S. Congress. The U.S. Senate may also remove a President from office only if two-thirds of the Senators vote to do so; a simple majority will not suffice.

In American political history, the Democratic Party used a two-thirds rule to choose its presidential candidates until 1936. In order to win the party's presidential nomination, a candidate had to be approved by two-thirds of the delegates to the party's national conventions; a simple majority of the delegates was not sufficient to win the nomination. The rule was created to give Southern Democrats a "veto power" over the nomination, and often worked to prevent the nomination of liberal candidates, especially on issues regarding civil rights for black Southerners. However, an unintended consequence of the rule was that potentially strong candidates were often eliminated, and instead weaker "compromise candidates" were nominated. The rule nearly prevented Franklin D. Roosevelt's nomination at the 1932 Democratic Convention. After he won the Presidency, Roosevelt used his prestige to end the two-thirds rule and since the 1936 election all Democratic presidential candidates have been chosen by a simple majority vote.

The precise definition of "two-thirds" varies as to whether this includes those explicitly abstaining, those passively abstaining and those not present. Among organizations that have adopted Robert's Rules of Order as their parliamentary authority, "two-thirds" is understood to mean two-thirds of those present and voting, with abstentions not counted as votes on either side. A measure would pass a two-thirds vote if the Aye votes were at least twice as many as the No votes.