That's how it's done in my country, France: candidates can't pay for political ads. They have equally-sized and free-to-use billboards in front of public buildings [0] and a free one-minute-and-a-half-per-day ad space on public television [1]. That's it.
[0] http://dspagnou.celeonet.fr/wp-content/uploads/2012/04/degra...
[1] For fun, the 2017 ads: https://www.youtube.com/watch?v=n__Il8Z7lrg