Yep. I'd put it as zero with Clinton, non-zero but statistically insignificant with Trump, and 25% with Obama.
I'm not sure but I can imagine him saying that at some point. Still, I'd put him at non-zero because he doesn't have the same political baggage that Clinton would worry about.
In general, Obama had a bad run because he did a pretty bad job, so his legacy is shot. Clinton is no small part of his failures with the Benghazi incident, Libya in general, Egypt, Ukraine, and the asinine "Russia reset". Clinton's campaign is disintegrating, so I think she'll fade before the election.
Trump is a complete mystery. He could be a totalitarian, using Obama's extra-constitutional maneuvers as a springboard to dominate the people. Or, he could be a populist and flop around whichever way the wind blows - he could pardon Snowden just to placate the mob and get some short-term bump in approval ratings - or just as easily put a hit on the guy to get the same.
When Trump is not adjusting his message for an audience, I get the sense he thinks he can govern like a big city boss does. In the best case scenario, I envision Trump will be unable to govern because capitol hill won't respond to his small ball policies, belligerence, and whining. Congress will just plow forward with their profligate debt spending and toss him a policy bone from time to time.
Either way it's not going to happen till after the election and most likely not till the end of his term in January. I'd expect it to be more likely for him to be pardoned if Trump is elected as otherwise Clinton, by association, would be dogged with "granting quarter to an enemy of the state".