And Romney did not win primaries by the large margins that Trump is winning them. Romney was shoved down people’s throats; nobody (except the GOPe and mormons wanted him.) As Romney destroyed the other candidates with his lies, their supporters did not move toward Romney, they moved to another candidate.
In order to win the nomination, you have to win at least 50% of the delegates. So far, of the total number of delegates awarded, Trump has won 46%. So, if Trump continues to win primaries by the same "large margins" then the nomination will be decided at the convention, where all bets will be off.
This of course is why Trump supporters are starting to make the false argument that as long as Trump has more delegates then any other candidate, not giving him the nomination will be stealing it from him. Apparently, almost only counts in horseshoes, hand grenades and if Trump cannot win the required number of delegates.