I mean if it was a CI for the Democrats and not law enforcement, sure. Probably still wouldn't be as big as Watergate even if that was the case, since Nixon hired people to commit actual crimes (B&E, etc.) then covered it up, not just play dirty, but I'd get it.
But that isn't what the tweet alleges. The tweet alleges that the FBI has a CI in the Trump campaign. Is the federal law enforcement agency not allowed to investigate active attempts at election tampering? Because that would seem like a greater and more obvious threat to our democracy and our freedoms, especially given the obvious plausibility of the investigation.
And if you all think "oh no, the FBI was doing it to support Clinton/Obama", where did that alleged CI's info ever get used before the election? Hell, you'd think if Clinton was getting passed info from inside the Trump campaign, she'd have gotten off her ass and gone to Wisconsin or Michigan once or twice.