Presidents have been usurping authority for a long time. Did Kennedy have authority to send "advisors" to Vietname? If he did, Johnson almost certainly didn't have authority to begin *his* military buildup without a heckuvalot more Congressional approval. And we know Nixon did quite a bit of trampling himself. Bush, and by association Reagan (although there's no hard proof) is thought to have been involved in the Iran-Contra deals. And on and on.

Maybe Ford and Carter were the last two non-power grabbers? Ford, because he didn't last that long in office, Carter maybe?