The petition signatures of a campaign to legalize limited possession of marijuana in Colorado need a line-by-line review, the secretary of state’s office announced Thursday.

Earlier this month, the campaign turned in nearly 160,000 signatures in an effort to put the measure on this year’s ballot. The proposal needs only 86,105 valid signatures to qualify.

When secretary of state staffers began reviewing a sample of the signatures, though, they did not find enough valid ones to be able to project with certainty that the measure would meet the threshold. The office projects, based on the sample, that the measure will have about 89,000 valid signatures — or 103 percent of the needed amount. State law requires a projection of 110 percent or more of the needed amount to certify a measure without a signature-by-signature review.

The office now has until Feb. 3 to verify the petition’s signatures.

Even if the campaign comes up short in the new count, it will have an extra 15 days to get the additional needed signatures, said campaign proponent Brian Vicente.

“We’re confident that we will ultimately qualify,” Vicente said.

The proposal seeks to legalize possession of up to one ounce of marijuana for any purpose for those 21 and older. It would allow for marijuana stores but it also would allow communities to ban such businesses.

The measure is backed by a number of prominent national drug-reform advocacy groups.

This week, a different group of activists backing a separate legalization proposal — one that would prohibit judges from imposing penalties on anyone for marijuana possession of any amount — received the required go-ahead from a state board. They may soon begin collecting signatures.

A third group of marijuana activists has said it also intends to file a legalization initiative.

John Ingold: 303-954-1068 or jingold@denverpost.com