United States Code, 2018 Edition
Title 3 - THE PRESIDENT
CHAPTER 1 - PRESIDENTIAL ELECTIONS AND VACANCIES
Sec. 8 - Manner of voting
From the U.S. Government Publishing Office, www.gpo.gov
§8. Manner of voting
The electors shall vote for President and Vice President, respectively, in the manner directed by the Constitution.
(June 25, 1948, ch. 644, 62 Stat. 674.)