In April 1816 he ran for Governor of New York and lost to Daniel D. Tompkins. In the fall of that year, he did become the informal presidential nominee for the Federalist Party, as they did not meet for any convention. He only received 30.9% and lost again, this time to James Monroe, whose running mate, coincidentally, was Tompkins. King would be the last presidential candidate by the Federalists before their collapse. When he ran for re-election to the Senate in 1819, he ran as a Federalist even though the party was already disbanding and had only a small minority in the New York State Legislature. But, due to the split of the Democratic-Republicans, no successor was elected to the U.S. Senate in 1819, and the seat remained vacant until January 1820. Trying to attract the former Federalist voters to their side at the next gubernatorial election in April 1820, both factions of the Democratic-Republican Party now supported King, who served another term in the U.S. Senate until March 3, 1825. At the time the Federalist Party had already ceased to exist on the federal stage. During his second tenure in the Senate, he continued his career as an opponent of slavery, which he denounced as anathema to the principles underlying the Declaration of Independence and the Constitution. In what is considered the greatest speech of his career, he spoke against admitting Missouri as a slave state in 1820.