With the possibility of a contested convention in Cleveland very much a live option, the rule that has attracted attention, known as Rule 40(b), was adopted at the party’s 2012 convention and requires a candidate to win a majority of delegates in at least eight states to be considered for the nomination.
http://www.washingtontimes.com/news/2016/mar/31/rncs-2012-rule-disqualifies-anyone-other-than-dona/