Campaign statements are rarely outright lies; there are half-truths, evasions, and statements that are true but highly misleading. For example, imagine a bill that extends unemployment benefits but provides an enormous corporate tax cut, which motivates a Democratic congressman to vote against this bill, but he later votes for a bill that extends unemployment benefits without cutting corporate taxes. The statement "CONGRESSMAN BOB VOTED AGAINST EXTENDING UNEMPLOYMENT BENEFITS" is then an entirely true statement, as the congressman did, indeed, vote against a bill that extended unemployment benefits. So this statement could not be punished, even though it is highly misleading. However, it is entirely possible that the statement "BUSH LIED US INTO WAR" could be punishable, as while we all know that the Bush administration spun an insidious web of deceit regarding the Iraq invasion, it is hard to point to a single, actual, provable "lie" from George W Bush regarding this.

There is a reason that the Washington Post fact-checker does not judge campaign claims to be "true" or "false" but rather awards between one and four "pinnochios". A law that punishes "lies" provides awesome censorship powers for whoever has the authority to judge whether a statement is the "truth". And it is extraordinarily naive to think that conservatives would not horribly abuse such a law.