Obama can/would be impeached by the House if he attempted a whilesale amnesty, to the extent of his pardon powers as High Crimes & Misdemeanors are for Congress to define, ultimately. I hope he does something blatantly foolish. He and the left will unleash fury like they have never seen.
Actually, Obama is pretty much putting de facto amnesty in place by refusing to enforce existing immigration laws and to extend government money and entitlements to illegal immigrants.