Political parties at every level of government choose their nominees through primaries. That's the most important decision a party can make—and an organization's most important decisions should be made by members of that organization.
When considering Trump's behavior, it is important that we distinguish between those actions that are simply policy preferences and those that are, in fact, illiberal.
For an alternate party response to an unsavory candidate's nomination, look to the actions taken by Colorado Republicans during the 2010 governor's race.
Donald Trump will deliver his first State of the Union address tomorrow. Last year's (well-received) speech before Congress can serve as a blueprint for what we can expect to hear.