Debates tend to be held at most countries right before elections. However, these are mostly traditions, and not mandated by law. For example, Romania got some criticism for not having enough of these, while in Hungary Orbán had his last debate back in 2006 (against Gyurcsány who won that time), and other countries might have similar problems.
In the US, according to the relevant wikipedia page, the constitution doesn't mandate debates. Is there any law about it, either about form (e.g. length, number of topics), or simply about its existence, or is this just tradition there as well? What happens if candidates can't agree on the rules?