ATLANTA — Voters across Georgia are deciding whether to levy a penny sales tax to fund transportation projects in their communities.

Supporters have spent $8 million trying to convince voters that the plan will add jobs, ease congestion and improve the quality of life. The plan has been endorsed by Republican state leaders including Gov. Nathan Deal, Lt. Gov. Casey Cagle and House Speaker David Ralston, in addition to Atlanta Mayor Kasim Reed, a Democrat.

Critics blast the plan as not only the heftiest tax proposal in state history, but as a false strategy that addresses neither sprawl nor smart growth.

Tea party members, the state NAACP and the Sierra Club comprise an unlikely coalition that opposes the referendum. They have been using e-mail and social media to urge voters to defeat the measure.

A dozen regions will vote on Tuesday's referendum. If a majority in a region votes in favor of the referendum, it passes there — even if other regions defeat it. Money generated by the tax would be spent in the region. Regions that do not pass the referendum get nothing.

If passed in all 12 regions, the tax would generate more than $18 billion to pay for transportation projects statewide over the next decade.

Polls opened at 7 a.m. and are scheduled to close at 7 p.m.

In metro Atlanta, supporters estimate an economic impact of more than $8.4 billion between 2013 and 2022. The 10-county metro Atlanta region stretches from Cherokee to Fayette counties and includes Gwinnett, DeKalb, Fulton and Cobb counties — among the state's most populous.

Regional commissions gathered public input for months before coming up with local project lists of varying scale and budget. For instance, more than $112 million would be used to reconstruct the interchange of Interstate 285 North and Georgia 400. Another $59 million would help pay for a widening project in the heart of the state that officials say would boost economic development and regional employment.