What are the First Amendment rights of donors in support of referenda?

In California, they say that the opponents of the recently-passed referendum to re-outlaw same-sex marriage are tracking down those that gave money to advocacy groups in support of the measure and trying to make them regret their position.

So, this lawsuit has been filed, claiming that the California law requiring disclosure of such contributions is a violation of the First Amendment rights of the donors, particularly because none of the rationale for intruding on the speech rights of donors in the usual campaign setting applies when there are no candidates to be corrupted by the cash.

I can understand that aspect, although I think the outcome of the suit depends on whether the issue comes down to standard of review - is this an incursion on First Amendment rights which must be narrowly-tailored or merely something that requires a rational basis. This I wonder.