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

State Control of Electors

There is no federal law that requires electors to vote as they have pledged, but 29 states and the District of Columbia have legal control over how their electors vote in the Electoral College. This means their electors are bound by state law and/or by state or party pledge to cast their vote for the candidate that wins the statewide popular vote. At the same time, this also means that there are 21 states in the union that have no requirements of, or legal control over, their electors. Therefore, despite the outcome of a state’s popular vote, the state’s electors are ultimately free to vote in whatever manner they please, including an abstention, with no legal repercussions. The states with legal control over their electors are the following 29 and D.C.:

Alabama (Code of Ala. §17-19-2)
Alaska (Alaska Stat. §15.30.090)
California (Election Code §6906)
Colorado (CRS §1-4-304)
Connecticut (Conn. Gen. Stat. §9-176)
Delaware (15 Del C §4303)
District of Columbia (§1-1312(g))
Florida (Fla. Stat. §103.021(1))
Hawaii (HRS §14-28)
Maine (21-A MRS §805)
Maryland (Md Ann Code art 33, §8-505)
Massachusetts (MGL, ch. 53, §8)
Michigan (MCL §168.47)
Mississippi (Miss Code Ann §23-15-785)
Montana (MCA §13-25-104)
Nebraska (§32-714)
Nevada (NRS §298.050)
New Mexico (NM Stat Ann §1-15-9)
North Carolina (NC Gen Stat §163-212)
Ohio (ORC Ann §3505.40)
Oklahoma (26 Okl St §10-102)
Oregon (ORS §248.355)
South Carolina (SC Code Ann §7-19-80)
Tennessee (Tenn Code Ann §2-15-104(c))
Utah (Utah Code Ann §20A-13-304)
Vermont (17 VSA §2732)
Virginia (§24.2-203)
Washington (RCW §29.71.020)
Wisconsin (Wis Stat §7.75)
Wyoming (Wyo Stat §22-19-108)

Most of these state laws generally assert that an elector shall cast his or her vote for the candidates who won a majority of the state’s popular vote, or for the candidate of the party that nominated the elector.

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. In fact, in the 2000 election, Barbara Lett-Simmons, an elector for the District of Columbia, cast a blank ballot for president and vice president in protest of the District’s unfair voting rights. Indeed, when it comes down to it, electors are ultimately free to vote for whom they personally prefer, despite the general public's desire.