Supporters of a referendum that would expand MARTA to Clayton County were jubilant Tuesday after returns throughout the night showed the measure passing by a wide margin.

Pro-transit advocates proclaimed a historic victory at the Friends of Clayton Transit election watch party in Riverdale, where a crowd of more than 100 volunteers, organizers, politicians and MARTA officials gathered.

Clayton is now poised to become the first new county to add MARTA since the agency began operating in DeKalb and Fulton in 1971.

“People recognized the opportunity to increase travel options was something they wanted to say yes to,” said Colleen Kiernan, director of the Sierra Club’s Georgia Chapter.

Of metro Atlanta’s five core counties, Clayton is the only one that lacks a local public transit system. A county-operated bus system, C-Tran, was dissolved in 2010 due to lack of funds. MARTA supporters hope the transit line will spur economic development and revitalization.

The vote means Clayton’s sales tax will rise from 7 percent to 8 percent, starting in March. The MARTA tax is expected to generate proceeds of about $45 million per year.

Half the money will finance limited bus service starting in March and full bus service the following year. The other half will be set aside for a future commuter rail or a comparable form of high-capacity service (such as bus rapid transit).