Except for actually voting and running for elected office themselves, the laws for permanent residents in the political process are the same as for citizens. So in this case, it's not a question of non-monetary versus monetary contributions—Rapi can also legally write a check to the Sanders campaign because he is legally an American national. Getting the green card is the point when someone is no longer considered a foreign national.

I personally participated in the 2008 Democratic caucus as a permanent resident. I could take part in the political discussions, but I just couldn't vote (which is a party rule, not a law). I also lobbied state legislators on behalf of the school I was attending.

http://www.fec.gov/ans/answers_general.shtml#Can_nonUS_citiz...