Oh, and one more thing: the McCain-Kennedy bill (as well as its successor bills) would constitute an un-Constitutional ex post facto law, as it would have retroactively changed the penalties (punishments) for unlawful presence by a foreign national on United States territory. Such laws at the Federal level are explicitly forbidden by Article I, Section 9 of the United States Constitution.
Nope. You can change punishments without a law being ex post facto.