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All the elements for swiftly legalizing marijuana in New Jersey seemed to be in place: A proposed bill was enthusiastically backed by Gov. Philip D. Murphy and had been endorsed by leaders of the Democratic-controlled State Legislature. Also, statewide polls showed support for the issue.

Then the plans unraveled.

Some lawmakers were unsure about how to tax marijuana sales. Others feared legalization would flood the state’s congested streets and highways with impaired drivers. Some would not be deterred from believing that marijuana was a dangerous menace to public health.

A disagreement existed among lawmakers about how far to go regarding the social justice component in the legalization bill: Fissures grew over whether it was necessary to expunge criminal records for marijuana-related offenses for those found with as much as five pounds of the drug.

“Five pounds is a lot of pot,” said Christopher Bateman, a Republican senator who had been considered a swing vote but had come out recently opposing legalization. “That’s not you and I smoking a little bit. That’s a dealer on the corner, selling and distributing.”