For collaborating with a foreign power in attempting to influence US policy. Here's a former director of AIPAC saying just that (in, mind you, the Jewish Daily Forward): https://forward.com/opinion/395676/its-time-for-aipac-to-reg...

The German American Bund was also an American organization made up of Americans ... and getting its marching orders from abroad in service of a foreign regime. That's why we passed the FARA law in 1938 in the first place. Foreigners are not supposed to influence US elections. Everything you have said about AIPAC applied to the German-American bund when they passed the FARA law. Neither the NRA or the American Cancer Society are advocating for policies for benefit of a foreign power: the German American Bund used to do this, just as AIPAC does today. If you don't like FARA, lobby to get it revoked. I'm sure that will go over well after the Russia hysteria.

There's also the example of the Lawrence Franklin espionage scandal of 2003 for shady AIPAC doings beyond this:

https://en.wikipedia.org/wiki/Lawrence_Franklin_espionage_sc...

AIPAC were pressured to register as foreign agents back in 62, but Kennedy being shot kind of pushed that aside. Ended pressure on them to stop their nuke program as well.

https://www.israellobby.org/azcdoj/default.asp