A campaign to legalize limited possession of marijuana in Colorado says it plans to turn in more than 12,000 additional signatures Friday in a last-chance effort to make the ballot this year.

Earlier this year, the campaign turned in more than 163,000 signatures in the hopes of putting its initiative on the ballot. Initiatives must gather 86,105 valid signatures to go before voters. After the Secretary of State’s office went line-by-line through the marijuana campaign’s petitions, it counted only 83,696 valid signatures.

The campaign was then given a two-week grace period to collect the extra 2,400 signatures to make the ballot. If it fails, its supporters would have to start the initiative process from the beginning.

The proposed initiative would legalize possession of up to 1 ounce of marijuana in Colorado for people 21 and older and would allow adults to grow up to six marijuana plants in their homes. It would establish a regulatory framework for pot shops — which could sell large quantities of marijuana — but it would also allow communities to ban the businesses. It would also allow for the growing of industrial hemp.

All those activities would remain illegal under federal law.

The proposed initiative is one of three marijuana-legalization measures working toward the 2012 ballot. The other two, which have not turned in petition signatures yet, would legalize unlimited marijuana possession for adults.

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