Now that a proposed penny sales tax to fund billions in transportation projects in metro Atlanta has failed, solutions to easing gridlock in the South’s economic engine are farther away than ever.

A day after Tuesday’s primary election, supporters vowed to change the minds of voters who rejected the ballot issue by a 2-to-1 margin. Critics derided the plan as an unfair tax on the poor that doesn’t really address sprawl and could not be entrusted to state government.

Meanwhile, the Atlanta region’s population — which swelled by more than 4 million people in the past four decades — continues to grow, fanning out farther from the capital’s core.

Observers say the governor and lawmakers will have to come up with an answer and the fix will likely not be easy.

“This is a promising opportunity that slipped through their fingers,” said Adie Tomer, a policy analyst with the Brookings Institute, a Washington-based think tank. “When the next opportunity will come in such an enormous, one swoop to address these issues, is questionable.”

Just getting the issue to a ballot was a four-year process: The Transportation Investment Act of 2010 was billed as an issue of local control by the GOP-controlled Legislature, which passed the decision to raise taxes on to voters, rather than deal with the issue themselves. While the referendum passed with bipartisan support, lawmakers scattered when it was time to urge voters to support it.

Metro Atlanta had the most to gain from the plan and organizers spent $8 million trying to sell the tax to voters. The 10-county region was expected to raise $8.4 billion over the next decade on dozens of road and transit projects supporters said would create jobs, ease congestion and improve frustrated commuters’ quality of life.

Unofficial results from the July 31 primary showed 63 percent of voters rejected the plan, with only 37 percent supporting it. It failed by a much larger margin than anticipated in all 10 counties making up the metro Atlanta region. While the vote was closer in Democratic-leaning and majority-black counties like Fulton, DeKalb and Clayton, it was no contest in largely white and Republican-leaning counties like Cherokee, Cobb and Gwinnett. Read more: