With the deadline six weeks away, organizers of an initiative to repeal the Sonoma-Marin Area Rail Transit sales tax have about half the signatures they believe are needed to put a measure on the ballot.

Clay Mitchell of Windsor, co-chairman of Repeal SMART, said about 100 volunteers have returned petitions, containing 7,500 signatures.

Mitchell said an additional 119 volunteers have petitions that have not been turned in.

"I don't have a count at this point; I couldn't even guess," Mitchell said. "It is a gargantuan task."

RepealSMART believes that 15,000 signatures are necessary, citing the threshold set by Proposition 218, a measure passed by voters in 1996 that required two-thirds votes on most tax measures and set up an initiative process to repeal existing taxes.

Two other state election laws, however, have formulas that would set the thresholds at 30,000 or 39,000 signatures.

Mitchell acknowledged Friday that his group would not be able to meet those higher thresholds.

SMART supporters said they aren't surprised.

"I didn't expect the repeal people to be successful," said Windsor Mayor Debora Fudge, a member of the SMART board. "Everywhere I go I have heard support."

RepealSMART has been circulating petitions since late September in an effort to put a measure on the ballot next year to overturn Measure Q, the quarter-cent sales tax that was approved by 70 percent of Sonoma and Marin voters in November 2008.

In the three years since then, SMART has seen the sales tax revenues shrink in a bad economy and have been forced to scale back the commute rail line it initially can afford.

SMART now proposes a $360 million spending plan to launch commute train service from downtown Santa Rosa to downtown San Rafael, extending the line to Cloverdale and Larkspur when additional funds become available.

The beginning of service is likely to be delayed one to two years, to 2015 or 2016.

SMART's critics contend that is not what was promised voters. They say they don't believe there will be additional money for the extensions and that train service will not reduce congestion and vehicle emissions.

RepealSMART had a six-month window — until Jan. 27 — to circulate petitions and then submit them to the registrars of voters in Sonoma and Marin counties for verification before they are delivered to SMART.

It would be up to the SMART Board of Directors to accept the signatures and call for an election, which most likely would be in November. A simple majority yes vote would be necessary to overturn the tax, even though it required more than two-thirds for approval.