I’m not saying that Carlson is an actual active Russian agent, I’m just saying that it’s hard to think of what he’d be saying if he were said agent, that he isn’t actually already saying .
If he isn’t a paid Russian agent, he’s sure doing a lot of unpaid volunteer work for Putin.
Well, there is the term "agent of influence", a term I think is used by both sides (all sides, in fact) in the spy business.
Which doesn't mean someone is a direct employee of a foreign nation's intelligence services, it just means they are someone who can, for whatever reason, be counted on to take a position useful to the government concerned, due to having overlapping interests. An example would be (depending on the global food trade situation) the Russian government might count American wheat farmers as such, on the grounds that those farmers have an economic interest in being able to sell their products to Russia, so could be counted on to oppose sanctions that would prevent them doing so.
Trump and his supporters could maybe be labelled as such, without requiring them to actually be taking orders from Moscow.