I believe they are referring to the not ratified Equal Rights Admendment. It was the last one voted on and passed by Congress but the required number of states did not ratify it even though they extended the ratification time limit.
Since conservatives opposed the ERA it wouldn't fit. My thought was either an insinuation that conservatives are so stupid that they don't know how many amendment there are or a poor reference to the proposed Federal Marriage Amendment, or maybe flag burning, or maybe he can't read roman numerals and was referring to the 18th amendment. Who knows.