If Clinton is elected, they'll look forward to four years of endless hearings, investigations, and denunciations. They wouldn't have nearly as much scope for that sort of thing with Kaine in the White House.

The parallel with abortion is that the Republicans want to have something or someone to run against. They've been screaming about abortion for decades now. They use it to fundraise and to motivate the base. If they ever achieved what they say they want, they'd lose that issue.