two-thirds rule

[ too-thurdz ]
/ ˈtuˌθɜrdz /

noun

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.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019