Yes, non-profits may engage in advocacy, although there are restrictions.  There is also a legal difference between advocacy and lobbying. 

Advocacy is voicing support for an issue or cause, such as telling the public about the benefits of arts education on your website. This can include attending public meetings with the city government, such as budget hearings, and giving testimony. There are no limits to pure advocacy by nonprofits.

Lobbying (sometimes called “direct advocacy”) refers to advocacy efforts intended to influence legislation, such as writing to the Board of Supervisors to oppose a bill that affects arts education. Nonprofits may engage in a limited amount of lobbying; in San Francisco, a proposition enacted in 2015 places some restrictions on lobbying. See https://ballotpedia.org/City_of_San_Francisco_Registration_Fee_and_Monthly_Reports_for_Expenditure_Lobbyists,_Proposition_C_%28November_2015%29.

Engaging in significant advocacy/lobbying activities?

ABBA is focused on policy education and cannot provide legal advice, please follow your city and county guidelines when you consider participating in advocacy/lobbying activities. If your organization is spending more than 10% of its expenditures on direct advocacy/lobbying, you should file a 501(h) provision form. The current 501(h) provision allows nonprofit arts organizations to allocate expenditures of up to 20% of the first $500,000 of their annual budget to direct advocacy and lobbying. If the organization exceeds $500,000 in annual income, only 15% of the next $500,000 can go to advocacy expenditures, and so on up to $1 million. To engage the 501(h) provision, an organization must complete the IRS Form 5768 (PDF form).

More resources:

CalNonprofits website

Bolder Advocacy, Supporting or Opposing Ballot Measures in California: What You Need to Disclose?

Independent Sector, Advocacy Rules

American Bar Association, Nonprofits and Lobbying Yes, They Can!