Saul Levmore has posted this draft on SSRN. Here is the abstract:

The United States does not allow foreigners to influence U.S. elections by giving money to political parties or candidates, but many other countries do allow cross-border influencing. Indeed, the variety of policies among countries is remarkable. Some countries, like the United States, that forbid financial contributions from outside the country, do permit cross-border contributions across domestic borders, such as those of states and cities. Meanwhile, Canadians cannot contribute to the campaigns of U.S. political candidates, and cannot even (lawfully) influence the American political process by purchasing campaign t-shirts or bumper stickers. This prohibition is in place even though, like constituents of neighboring jurisdictions within the U.S., Canadians are greatly affected by U.S. policies