Who exactly would you say is maintaining party discipline? In 2012, Mitt Romney was at least nominally the leader of the Republican party as their Presidential

Who exactly would you say is maintaining party discipline? In 2012, Mitt Romney was at least nominally the leader of the Republican party as their Presidential nominee. Nowadays, Donald Trump is clearly attempting to maintain party discipline, but I don't think anyone has ever been able to maintain discipline over Donald Trump, not even before he was their President or Presidential nominee.