Hillary Clinton shouldn't wait till the primaries are over to start advertising on TV, nationally, as a hedge against Trump winning the nomination.
If it's Ted Cruz, nothing to worry about. He's smooth, but not so different from previous Republicans. He can't possibly attract Democratic votes, and without that he can't win. The Democratic Party is still the majority party, despite Congressional gerrymandering, and it's only becoming more so, even before Trump lit the political stage on fire with race fear. That just drives more minorities in the direction of the Democrats.
But if Trump wins, it's going to be important to have set the terms of the general election before he has a chance to do it, because he will win on that basis. He's very very good at manipulating the flow of discourse. He will pivot to stories that are appealing to disenfranchised people of all political parties. It's not that much of a pivot. He's appealing to Republican voters with a populist Soak-The-Rich platform. He's got that down. It's pre-built to work in the general election.
One ad, run during the Super Bowl, a mix of Apple's The Crazy Ones and Reagan's Morning in America. Americans are great, and we did a great job, dealt a shitty hand by an incompetent Republican predecessor. Why would we ever want to go back? Draw a positive picture of America, its genius and its power, and push aside all the doom and gloom of Trump.