Replies

http://history.house.gov/Institution/Electoral-College/Electoral-College/

Electors

Most states require that all electoral votes go to the candidate who receives the majority in that state. After state election officials certify the popular vote of each state, the winning slate of electors meet in the state capital and cast two ballots—one for Vice President and one for President. Electors cannot vote for a Presidential and Vice Presidential candidate who both hail from an elector’s home state.

The contested 1876 Presidential election brought Senators, and the electoral certificates under investigation, into the House Chamber.
Maine and Nebraska employ a “district system” in which two at-large electors vote for the state’s popular majority and one elector votes for each congressional district’s popular majority. In the November 2, 2004, election, Colorado voters rejected a “proportional system” in which electors would vote proportionally based on the state’s popular vote.

Thanks.
It seems 29 states and the Dist. of Columbia have such laws.
However, I also found this

http://archive.fairvote.org/?page=967

“Over the years, however, despite legal oversight, a number of electors have violated their state’s law binding them to their pledged vote. However, these violators often only face being charged with a misdemeanor or a small fine, usually $1,000. Many constitutional scholars agree that electors remain free agents despite state laws and that, if challenged, such laws would be ruled unconstitutional. Therefore, electors can decline to cast their vote for a specific candidate (the one that wins the popular vote of their state), either voting for an alternative candidate, or abstaining completely.”