Specifically, foreign nationals are not allowed to contribute money or things “of value” to campaigns, nor are Americans allowed to solicit such things. Here’s the applicable section of U.S. law:

“It shall be unlawful for . . . a person to solicit, accept, or receive a contribution or donation described in subparagraph (A) or (B) of paragraph (1) from a foreign national.”

Subparagraph A describes “a contribution or donation of money or other thing of value . . . in connection with a Federal, State, or local election.”

Mueller opted not to charge Trump Jr. for accepting a meeting in which he expected to receive opposition research on Clinton and the Democrats from a Russian lawyer in 2016. But it was not because there was nothing potentially wrong with this; it was because Mueller decided it was unlikely that it could be proven Trump Jr. knew that it was illegal and/or that the information was something “of value” — a key clause in the above legal code. Trump Jr. has always maintained that the information offered by a Kremlin-tied lawyer, Natalia Veselnitskaya, was useless.