That would be the Electoral College. Gov. Rod Blagojovich (D) signed a bill into law committing Illinois to a one-citizen, one-vote regimen, rather than the current winner-take-all regimen under the Electoral College system. According to watchdog National Popular Vote, it may be possible to replace the College with a popular vote when states representing over half of the Electoral College votes ratify the change. To date, 46 out of 538 have done so, with 21 from Illinois joining 15 from New Jersey and 10 from Maryland.
The meaning for broadcasters is clear, as noted by NPV: "Under the winner-take-all rule, candidates have no reason to poll, visit, advertise, organize, campaign or worry about the concerns of voters of states where they are safely ahead or hopelessly behind."
A total of 16 states are said to have passed popular vote legislation.
RBR/TVBR observation: You saw the word buried in the list: "advertise." Whatever you opinion of the Electoral College, its elimination will certainly bring every last state into play. Even the reddest red states and the bluest blue states generally have pockets where the loyal opposition holds sway. We’d like to see this change because we feel the College is anachronistic and anti-democratic. So much the better that it would be good for business.