Simmering opposition in south DeKalb County to the proposed regional transportation sales tax is heating up, posing another challenge to backers of this summer’s referendum.

The DeKalb NAACP has joined some politicians in opposing the proposed one-cent sales tax because the project list does not extend MARTA rail into south DeKalb.

Tax proponents say the opposition is shortsighted and that the tax-funded projects will boost south DeKalb along with the rest of the region.

DeKalb, along with Fulton and Clayton, are considered strongholds by supporters of the measure, with a poll last September bearing that out. The tax would raise $6.14 billion for a 10-county metro region, with more than half of it going to public-transit projects.

John Evans, head of the DeKalb County NAACP, said it is unfair that county residents have been paying a one-cent sales tax to fund MARTA, yet the biggest transit proposal in decades leaves off rail in the I-20 corridor despite years of planning for it. He said the group will campaign against the sales tax.

“We’re saying you have screwed us for years and we’ve paid all this money and you won’t even give us this rail to line to Stonecrest mall,” Evans said.

The big rail projects on the project list — approved by regional political leaders — include $600 million for the Beltline and streetcar project in Atlanta and $700 million for the Clifton Corridor project in northeast Atlanta and central DeKalb. South DeKalb would get a $225 million express bus service.

“Its going to be very difficult to pass,” said DeKalb County Commissioner Lee May, who represents southeast DeKalb. “My proposal is to vote this referendum down and start over again with another project list that treats DeKalb County in a more equitable manner.”

Next month, tax advocates roll out an “education” program to explain to voters what transit and road projects they will get from the 10-year tax.

DeKalb CEO Burrell Ellis, a tax supporter, said he expects a “healthy debate” with south DeKalb opponents but contended the county is being treated fairly, with nearly $1 billion going to DeKalb-related transit projects.

He noted that the Clifton Corridor rail project is half in DeKalb and will bring workers to the job centers around Emory University and the Centers for Disease Control and Prevention. South DeKalb would get all-day express bus service designed to connect residents to job centers such as Hartsfield-Jackson International Airport and the Cumberland Mall area.

The money can still be used for a rail line if the county and MARTA can secure scarce federal dollars, he said.

“We’re laying a foundation to bring job centers to south DeKalb, so for those people who say no, my question to them is: ‘How will we ever bring rail to south DeKalb and what is your plan?’ ” Ellis said.

“If this referendum fails, there is no Plan B,” Ellis added. “There is no other source of funds for that east line in South DeKalb. It could be another 40 years before it receives anything.”

But Commissioner Larry Johnson, who represents southwest DeKalb, said an I-20 rail line in one version or another has been promised for decades. Currently, proponents are seeking a 7-mile line from the Indian Creek MARTA rail station to the Wesley Chapel Road-I-20 intersection, which could be expanded to the Stonecrest mall area.

“A lot of folks are saying they couldn’t support the sales tax because we are tired of being out of the loop,” Johnson said. “We’ve been waiting 20 years and now you want to have us pay another penny sales tax.”

Sales tax backers are counting on a heavy favorable vote in Fulton and DeKalb to offset votes in other counties where resistance appears stronger.

Kerwin Swint, a political scientist at Kennesaw State University, said proponents face a battle.

“It will tend to be people who are motivated and have a stake in the outcome who show up,” he said. “I suspect you are going to have a pretty strong anti-tax showing.”

Cheryl King, assistant general manager for planning at MARTA, said Friday that the transit agency has a plan to provide for I-20 rail that didn’t make it onto the project list for the sales tax.

But she noted that the transit agency would have to come up with nearly half of the $1.8 billion price tag to pay for rail from the Stonecrest Mall area to the Indian Creek rail station and for a rapid bus service from Wesley Chapel Road to downtown Atlanta.

That assumes the rest would be federal money.

Jeff Dickerson, a consultant to the campaign to pass the referendum, said DeKalb voters need to think regionally and understand that projects funded by the special tax are projected to create 7,120 new jobs annually through 2040 and nearly $35 billion in economic activity over the next three decades, as well benefit commuting metro-wide.

“The folks in south DeKalb cannot reasonably say the I-285-Ga 400 interchange improvement doesn’t benefit them or the Beltline doesn’t benefit them,” he said. “The people in south DeKalb deserve a MARTA train. It is a question of what we are able to do today.”

DeKalb Commissioner Stan Watson contends the southern end of the county, hit hard by the recession and housing bust, risks turning into an economic wasteland unless rail arrives to spur economic development.

“We do have to take a regional approach to transportation but that region has to include DeKalb County and especially south DeKalb,” said Watson, who said he will decide in May whether to campaign against the referendum. “There is at least going to have to be some commitment to developing rail because I think the T-SPLOST can pass or fail in DeKalb County.”

The regional transportation vote

• When: Summer of 2012

• What: Voters in 10 counties will decide whether to impose a new tax for transportation improvements.

• How much: If passed, the tax will add a 1-cent sales tax.

• What it would buy: A roundtable of regional leaders approved a list of critical projects including highway and road widenings and transit lines. Local governments would get some of the money for their own projects.

• What’s next: Backers plan an “education” campaign starting next month to explain to voters in each county how they would benefit.