The main political campaign to legalize recreational marijuana in California spent months negotiating before finally unveiling its proposal last fall.

The high-stakes talks between deep-pocketed donors, drug-policy reformers, medical doctors, labor unions, environmentalists and many other groups representing professions with an interest in the process were so exhaustive that one observer, only in jest, compared the discussions to the Treaty of Versailles.

With the legalization measure aiming for November, and its would-be competitors faltering with little financial resources, old-school cannabis activists have slowly come to embrace the effort funded by billionaire venture capitalist Sean Parker and supported by Lt. Gov. Gavin Newsom.

On Monday, the campaign behind the Newsom-Parker legalization measure announced the backing of the National Organization for the Reform of Marijuana Laws. Allen St. Pierre, the group’s chief executive, said the significance of the statewide effort could not be overstated. 

“On the matter of ending marijuana...