Kerry is a "public figure", meaning there has to be actual malice to sue for defamation I believe.
That's nearly impossible, since no one can establish who is telling the truth, Kerry or the Swift Vets.
It's he said/they said.
No libel, no slander, no defamation under the law.
Kerry is trying to destroy the First Amendment and free speech in this country. He is dangerous.
In writing about trials and arrests, make sure you check the record. Before the person is found guilty, always use words like "alleged" or "accused." In America, everyone is considered innocent before proven guilty.
You are right, it is EXTREMELY difficult to find slander or libel on a public person, including politicians. Curious situation. Both sides have legal teams. Where's a lawyer when you need one--- to explain this! ;)