Donald Trump cannot unite the republican party, nor should he. The current Republican Party is really nothing short of democrats in sheep's clothing. There is no discernible difference between mainstream Republicans and Democrats. Donald Trump represents a new order for the Republican Party; One that will actually do what it says when they are elected, As opposed to pandering for votes.
Donald Trump will not be able to unite the Republican Party. The Colorado delegates took the unusual step of walking out of the RNC. Trump does not seem interested in making conciliatory statements and reaching out to the party. He has a combative personality and many do not seem eager to associate with him.
The Republican Party is divided, and Donald Trump is a part of the reason for that. As the party has moved further and further towards xenophobia, sexism, homophobia, etc. it has become more divided. Trump's campaign has finally put that division out in the open and emboldened people from both sides of the division to become more outspoken. Trump himself is a very divisive personality, and I don't think he will be able to unite the party, or even attempt to do so.
Donald Trump is like the obnoxious uncle we all just 'put up with.' Sure, he tells it like it is but with the aplomb of 5-ton wrecking ball in a china shop. In order for the Republican party to unite, many strong and competent leaders are going to need to step forward and reign Mr. Trump in or they can kiss the presidency goodbye.