Anti-Masonic party, American political organization that rose after the disappearance in W New York state in 1826 of William Morgan. A former Mason, Morgan had written a book purporting to reveal Masonic secrets. The Masons were said, without proof, to have murdered him, and in reaction local organizations arose to refuse support to Masons for public office. In New York state Thurlow Weed and William H. Seward attempted unsuccessfully to use the movement, which appealed strongly to the poorer classes, to overthrow Martin Van Buren and the Albany Regency. Anti-Masonry spread from New York to neighboring states and influenced many local and state elections. At Baltimore, in 1831, the Anti-Masons held the first national nominating convention of any party and issued the first written party platform—innovations followed by the older parties. The vote for their presidential candidate, William Wirt, mostly hurt Henry Clay. Usually the Anti-Masons in national politics acted with the National Republican party in opposition to Jacksonian democracy, and in 1834 they helped to form the Whig party.