Questions about a newly-formed super PAC supporting Teresa Tomlinson’s Senate campaign have triggered one of the sharpest clashes in the fast-approaching Democratic primary.

The clash emerged after a former campaign volunteer for Jon Ossoff asked the Federal Elections Commission to investigate whether Tomlinson's campaign improperly coordinated with Undivided Purpose, an outside group that shares digital consultants and fundraising advisers with her operation.

Then came a story from The Center for Responsive Politics, a watchdog group which asserted that the PAC is "keeping its donors hidden from voters and appears to have ties" to Tomlinson's allies.

The group, which has spent at least $91,000 boosting Tomlinson, initially didn’t file a pre-primary report with the FEC revealing its donors, though it did shortly after the center’s report was published.

Leading in the polls, Ossoff has largely avoided attacking Tomlinson or Sarah Riggs Amico, the other top Democratic contender in Tuesday’s primary for the right to challenge Republican Sen. David Perdue.

But, as recent surveys show him within reach of the majority-vote needed to avoid an August runoff, he leveled what is perhaps his first overt political attack on Tomlinson of the campaign.

“When a candidate has signed a pledge to ‘get dark money out of politics and live those values on her campaign,’ the solution for that candidate in this case is simple: she must denounce this unethical dark-money group immediately and call for its ads to come off the air,” he said.

Tomlinson, an attorney and former Columbus mayor, earlier said her campaign adheres to campaign finance rules, and her aide said Wednesday Ossoff is the “wrong person to wag his finger about so-called big-money” after his bid for Congress in 2017 attracted a record-setting $60 million in spending.

Tomlinson spokeswoman Nicole Henderson also said the criticism was a sign that Ossoff was growing increasingly concerned about her campaign, and referred to his recent cash infusion.

“If Jon Ossoff is so far ahead in this race, what does he care about a perfectly lawful Super PAC properly registered and apparently compliant with the law?” she said. “If Jon Ossoff is so far ahead, why has he had to transfer $450,000 to his own campaign?”

>>More: Ossoff applies lessons from 2017 loss to bid this year for U.S. Senate

>>More: Tomlinson says she's ready for the U.S. Senate, having already governed

>>More: In Senate run, Amico relies on experience in business, past election