Inside the Dark Money Boosting Bernie's Campaign

An outside dark money group has boosted presidential candidate Sen. Bernie Sanders since announcing his campaign last February.

Sanders founded Our Revolution, a 501(c)(4) nonprofit group that can accept unlimited contributions without disclosing the identities of its donors, in 2016, and one campaign finance expert believes the group’s work to help Sanders achieve his presidential aspirations could be violating the law.

“Any entity established by a federal officeholder can only raise and spend money under federal contribution limits for any activities in connection with a federal election,” Paul S. Ryan,...