Rubio and Cruz are taking nastiness to a new level, Trump is being dragged into the mud by them and Romney. Cruz is playing a voter suppression game that will only hurt the eventual GOP nominee. It is politics at the dirtiest level thanks mainly to Ted Cruz campaign manager. Trump only hits back in self defense.
Unfortunately many women and religious people don't understand that and they would rather stay home rather than vote for a guy who doesn't meet their standards of conduct.
Still, I wish Trump would tone some of his antics down. What works in New York City doesn't necessarily play well in Topeka, KS. I think it speaks to his relative inexperience in politics. Most presidential candidates have extensive political experience and know how to mask their rough edges while projecting a smooth and polished image to the voters.