What if there was a law that says all Presidential candidates during their campaign must ....?

....take an oath to tell the truth, the whole truth and nothing but the truth. If they are caught telling a lie or misrepresenting the truth, it is considered perjury and would be punishable by prison time and/or excessive fines, but more importantly they would be deemed untrustworthy and therefore unfit to be POTUS.

It seems like lying and being a politician is synonymous. Maybe that has been the norm for all these years because politicians could simply spin their words to defend their inaccurate statements. However, in the omnipresent media age that we live in today, where everything and anything they say could be recorded on Youtube, TMZ or camera phones, etc, they don't have that luxury anymore.

So, given this fact, shouldn't candidates for the most important position in the world be held to the highest standard. Do we really want to elect a POTUS just because he spoke mis-truths about himself or others and gullible people believed these lies and elected them based on that?

So, what do you think? Shouldn't this be a law? If not, why not?