Nate Cohn calls Clinton the "perfect surrogate":
Nearly all of the white voters without a college degree that Obama could plausibly persuade like Clinton and may have even voted for him in 1996. When you add Clinton's staggering personal popularity, it's hard to construct a better surrogate on the president's behalf: A figure of stature who retains immense credibility on the issue that endangers the president's reelection chances with the voters who may well decide his fate.
Andrew Sprung considers why the GOP's former presidents, besides Reagan, don't get mentioned:
It's not that Republicans haven't put up capable presidents. Its rather that the present party has moved so far to the right that it can only fully own one of its successes. Indeed, by contemporary standards, even Reagan was a flaming liberal, acceding to a long series of tax hikes to partially, incrementally offset the structural deficit he opened up with his original massive tax cut. That retroactive apostasy has yet to be fully absorbed, except by Democrats.
Larison argues that Republicans were wise to keep their distance from George W. Bush:
The only reason that Republicans ought to have mentioned him was to state their disagreements with what he did, but we all understand that this was never going to happen. The bigger problem for the GOP is that there is still so little in Bush’s record that they oppose. Even though they don’t mention him by name, the party still accepts most of the policies he favored.