I can't tell if you're intentionally stating falsehoods with the hope nobody will fact check you, or if you're just completely ignorant of the law. He was accused and convicted of acting as an unregistered foreign agent. Is it absolutely, unquestionably illegal to be a political operative for a foreign government and not register. For good reason. He broke the law, he went to prison. It was entirely illegal.
And NONE of that has anything to do with the original question or answer. Which is: what proof is there that he gave polling data to the Russians. Which I provided, and you still haven't refuted.
I assert strongly that no one on earth should care in the slightest.
Sharing non-classified information is completely legal. Working for a foreign business is legal. Getting paid is legal. Almost everything is legal.
Acting as an foreign agent without disclosure is illegal, in limited scope. The FARA violation related to Ukraine.
Fuzzy thinking around it was and is being used to excuse extraordinary abuse of the law and legal system.
To be illegal, information sharing has to violate some law. This does not. It is simply not illegal to share information with Russia citizens, the Russian government, or anyone affiliated with Russia.
Not even "secret" information, unless that information is governed under very specific laws. All this media puffery is absolute manipulation, and your distributing it shows that you either don't understand the law or don't wish to.