The imminent Trump nomination threatens to rip the Republican party into three parts. Trump repels both the most conservative Republicans and the most moderate: both socially conservative regular church attenders and pro-Kasich affluent suburbanites, especially women. The most conservative Republicans won’t ever vote for Hillary Clinton of course. But they might be induced to stay home—if Clinton does not scare them into rallying to Trump. The most moderate Republicans might well cast a cross party line vote—if Clinton can convince them that she’s the more responsible steward and manager.
Those have to be exciting possibilities for the Clinton campaign. For a generation, national politics has been polarized into two unified blocs with minimal cross-over. The Trump nomination suddenly makes imaginable an election like 1964 or 1972, in which a divisive nomination by one party propels millions of voters across the aisle to the other. But the way to win a 1964 or 1972-like victory is to move to the center, to position the winning candidate as the safe choice. Lyndon Johnson and Richard Nixon ran as candidates of peace and order, against destabilizing radicals.