3 U.S.C. § 3

The number of electors shall be equal to the number of Senators and Representatives to which the several States are by law entitled at the time when the President and Vice President to be chosen come into office; except, that where no apportionment of Representatives has been made after any enumeration, at the time of choosing electors, the number of electors shall be according to the then existing apportionment of Senators and Representatives.