The Sanders Institute, Bernie's think tank has stopped accepting donations.

[George II]

38. The Sanders Institute was founded as a 501(c)(3) organization. Their political activity....

...is extremely limited.

https://www.irs.gov/charities-non-profits/charitable-organizations/exemption-requirements-section-501c3-organizations

Excerpts:

"...it may not be an action organization, i.e., it may not attempt to influence legislation as a substantial part of its activities and it may not participate in any campaign activity for or against political candidates."

"Section 501(c)(3) organizations are restricted in how much political and legislative (lobbying) activities they may conduct."

"A section 501(c)(3) organization must not be organized or operated for the benefit of private interests, such as the creator or the creator's family, shareholders of the organization, other designated individuals, or persons controlled directly or indirectly by such private interests. No part of the net earnings of a section 501(c)(3) organization may inure to the benefit of any private shareholder or individual. A private shareholder or individual is a person having a personal and private interest in the activities of the organization."