Thanks to Trevor for the pointer.  Here are two relevant advisory opinions:

First, the FEC advised in 1987-25 that foreign nationals can volunteer for
campaigns:

http://ao.nictusa.com/ao/no/870025.html

"The Commission has concluded herein that because uncompensated volunteer
services are not considered to be a contribution under the Act, any
individual, including a foreign national, may volunteer his or her
uncompensated services to a candidate without making a contribution to that
candidate."

However, in 2004-26 the FEC advised that the activity cannot be in a role of
decision making:

http://ao.nictusa.com/ao/no/040026.html

"(a foreign national) may, as an uncompensated volunteer, take part in these
Committee activities as long as she does not participate in the Committees'
decision-making processes."  And "...may not, however, be involved in the
management of the Committees."

The question naturally arises: when is the line crossed where a volunteer's
activities rises to the level of decision-making?  Does the volunteer who
queries a database to put together a walk list and writes a walk script
enagage in decision-making (I've done this as a volunteer)?  How about the
volunteer who decides to visit a house not on the walk list and has a
conversation off the script (I've made this decision, too, while "managing"
a walking partner)?

------------
Dr. Michael P. McDonald 
Assistant Professor, George Mason University 
Visiting Fellow, Brookings Institution

                          Mailing address: 
(o) 703-993-4191          George Mason University 
(f) 703-993-1399          Dept. of Public and International Affairs
mmcdon@gmu.edu            4400 University Drive - 3F4 
http://elections.gmu.edu  Fairfax, VA 22030-4444