Clinton, as of now, has been running a predictable campaign against Trump. And she is using Ryan’s reluctance to endorse him to make the point that Trump is too extreme and risky even for his own party. But that plays right into Trump’s hands.
It allows him to argue that unlike her, he isn’t beholden to the whims of any special interests or Washington politicians, or blinded by partisanship. Instead, he’s interested in doing what’s right for the American people. He can tout, for instance, his willingness to buck the Republican orthodoxy on trade, entitlements, the minimum wage, and infrastructure spending.
Once he joins forces with Ryan, he becomes seen more as a cookie-cutter politician who is going to implement the GOP agenda, without the benefit of being able to really make a tangible difference in terms of unifying the party’s voters.
For Ryan, the reasons not to endorse Trump are quite numerous. Ryan has always prided himself as being a person who was inspired by figures such as Ronald Reagan and Jack Kemp to try to advance a set of ideas and offer serious policy solutions. He’s made a number of compromises along the way, but that goal has always been a central part of his message.