It was never formalized in the past but has always been an unspoken rule: if you’re in the party, you support the party’s leader. Otherwise, why are you in the party?
In 2016, Trump caused a big stir because he stated he would not support the candidate if it wasn’t him. Some people think this was part of the reason he won, that some voters chose to support him because if he lost and ran as a third-party candidate, it would almost guarantee a victory for the Democrats.