1. Libel is not criminal- not in America. It is in many other countries.
2. The 1st Amendment has many safeguards against libel lawsuits, and certainly libel causes of action have indeed been found unconstitutional. If you're "intentionally lying" against even a public figure, even the First Amendment won't be likely to help you, unless the plaintiff can show damages. Not many politicians bother to sue against lies, because, what's the point.
3. The concern about what Walz said, which I didn't know about until today, because I honestly had barely heard of him, is two fold.
(A) Hate speech is absolutely protected by the 1st Amendment.
(B) "Misinformation" is absolutely protected by the 1st Amendment.
I actually have no idea what kind of hate he wants to outlaw, but I absolutely know what kind of "Misinformation" he wants to. He wanted to hit that Ivervectim crap and anti vaccine crap (which may or not actually be crap, but whatever), and he wanted to hit it hard. But, you can't do that in THIS country. 1st Amendment, see.
4. To repeat, libel laws, which are themselves, in THIS country, are civil, not criminal. And laws against "hate speech" and "Misinformation" are not the same as civil common laws against libel, because they don't even exist.
6. In general circumstances, lying is ABSOLUTELY protected by the 1st. While it is allegedly a sin to tell a lie, it's no cognizance crime.