You are correct that 501(c)(3) charities may not support candidates for election to public office at any level of government.  (See Ready Reference Page: “IRS Guidance Has Not Changed on Electioneering”) But the IRS does not treat a ballot initiative as an election of a candidate for public office.  It treats taking a position on a ballot initiative as “lobbying” activity because the initiative can result in a law enacted directly by the voters.  So for most charities, engaging in lobbying is fine so long as it is not a substantial part of their activity.  (See Ready Reference Page: “Lobbying Rules Create Opportunities for Charities”)

But that doesn’t necessarily establish the answer to your question.  You have not said what kind of “foundation” you are talking about.  The term “foundation,” by itself, has no definitive meaning in the law.  It could be a public charity, like a community foundation or any of the numerous foundations dealing with diseases, which has a broad range of public support, or a private foundation, like the Gates Foundation or the Ford Foundation, whose only income comes from a very narrow source, such as single individual, family or company and from investment income.  (See Ready Reference Page: “Calculating Public Support Percentage”)

A public charity is allowed to engage in lobbying generally.  A private foundation would incur an excise tax for expenditures on lobbying generally, and is permitted to engage only in “self-defense” lobbying on issues that would affect its continued existence, its powers and duties, or deductibility of contributions to it.  If Congress sought again to increase the annual private foundation payout requirement from 5% to 7% of its net investment assets, opposing that legislation would be considered self-defense lobbying.  It is hard to see how lobbying on the legalization of marijuana would fit within the definition of self-defense lobbying by an ordinary private foundation.