A Dunwoody attorney whose investment firm offered risk-free returns of 300 percent every 14 days misused at least $800,000 of investor money, according to a securities fraud lawsuit filed Thursday by federal regulators.

The defendant, Michael Rothenberg, is a former candidate for DeKalb County Superior Court Judge who was sued separately by an investor during his failed campaign. The new suit by the Securities and Exchange Commission contends Rothenberg transferred $210,000 in investor funds to his political campaign’s bank account, and used another $190,000 for personal expenses.

The SEC civil suit alleges Rothenbergtold investors he traded sophisticated international bank instruments.

The SEC said the scheme raised more than $1.7 million from at least two investors between February and March 2010. It said Rothenberg returned $910,000 to investors, but “misappropriated at least $800,000 of investor funds.”

Last year, Rothenberg and his company, Four Five LLC, were sued during the runoff by a Colorado-based investor that accused Rothenberg and Four Five of fraud. Rothenberg denied the allegations at that time. A $1.3 million judgment was issued against Rothenberg and Four Five in federal court in March, court records show.

Rothenberg said Thursday he was unaware of the SEC suit and declined further comment.