ATLANTA — 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 further 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 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.

An unlikely coalition including tea party activists, local NAACP leaders and the Georgia chapter of the Sierra Club helped defeat it. The groups spent far less money, but mobilized through grassroots organizing, email and social media.

Among the few Republican leaders willing to publicly back the plan was Gov. Nathan Deal, who inherited it from previous Gov. Sonny Perdue. Deal pitched it as a matter of economic competitiveness against other growing Sun Belt cities, like Charlotte and Dallas.

He lamented the referendum’s defeat Tuesday night, but said he would work with legislators to set priorities for projects worthy of state funding — a signal that the list will almost certainly shrink.

“Given state budget constraints, significant reductions in federal funding and the long time it takes to get projects completed, the rejection of the (transportation tax) significantly reduces our capacity to add infrastructure in a timely fashion,” he said in a statement.

Atlanta Mayor Kasim Reed, perhaps the most visible supporter of the referendum leading up to the vote, said he would continue trying to win over voters for another attempt at revising the project list for a future vote.

Clark Atlanta University political science professor William Boone said the mayor and other supporters seemed not to know their audience on this issue — especially in the African-American community.

Some black leaders thought the plan didn’t do enough for lower-income parts of the city, which continue to struggle with service cuts and fare hikes on MARTA, the region’s public transit system. It’s the largest transit system in the nation that doesn’t receive any state funding. Fulton and DeKalb counties have been paying a penny sales tax to support MARTA since 1971.

“They underestimated what the black community would say,” Boone said. “They were not interested in funding transportation projects that were once again not beneficial to them.”

Tomer said that while education is a good strategy, changing voters’ minds may not be the problem.

“It’s more an information campaign than anything else,” Tomer said. “There was a lack of knowledge about what this would do. There was a detailed project list, but it’s asking a bit much, with people’s lack of time, to expect them to flip through that.”

Even before the vote, supporters were skeptical that lawmakers would take another crack at reviving a referendum. Deal has said he is not interested in revisiting the referendum or funding transit.

“On public transportation, yesterday’s vote slams the door on further expansion of our rail network any time soon,” Deal said. “Neither I nor the Legislature has much of an appetite for new investments until there are significant reforms in how MARTA operates.”

Boone said ultimately, it would be up to lawmakers to stop shirking their responsibility and deal with the state’s transportation woes.

“The state Legislature has to look at the state as a whole, and not try to pawn this off to regions,” Boone said. “They have to find some way of doing this. They have abdicated their responsibility to the state.”

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