Easy, just revoke the most protected form of speech in the country during such times when that speech is most important. I don't think the Supreme Court is going to buy that one.
Why? Every candidate still gets some time-slots for stating their opinion. The point is that everybody gets the same amount of time.
And people who are not candidates and who want to express their opinion, they can: (1) vote, or (2) be a candidate. What they can't do is buy media time or advertisements, etc., which many wouldn't equate with "free speech" anyway.