In a report released Monday, the Office of Congressional Ethics also found Deal exceeded the limit on outside income he can earn as a member of the U.S. House.
The alleged violations might never be confirmed because Deal quit Congress earlier this month before the House Committee on Standards of Official Conduct could complete an investigation. Deal has said he resigned to focus on his gubernatorial campaign.
Deal spokesman Harris Blackwood blasted the conclusions of the nonpartisan office as "a political witch hunt fueled by Democrats" who think the ex-congressman from Gainesville would defeat Democratic favorite Roy Barnes in the race for governor.
"Nathan Deal did nothing wrong," Blackwood said.
Congressional investigators found that Deal made at least $75,000 in 2008 in outside income, well over the limit of $25,830.
Blackwood chalked that up to "an innocent accounting error" and said Deal had corrected the past three years of tax returns after learning of the mistake.
The report also alleges that Lt. Gov. Casey Cagle, also of Gainesville, refused to cooperate with congressional investigators who sought an interview. Cagle helped Deal coordinate meetings with state Revenue Commissioner Bart Graham, who was proposing changes to the way the state inspects rebuilt salvaged vehicles.
The report recommended the Committee on Standards of Official Conduct of the U.S. House subpoena Cagle to get him to talk.
"Today's report conveniently omits the dozens of interactions between my office and the OCE," Cagle said in a statement Monday. "We have voluntarily taken an active role in an investigation that we are not even the subject of and furthermore conducted by an organization that exercises no jurisdiction over state constitutional officers. Simply put, we've given them everything we have to give them."
The allegations against Deal were first reported by The Atlanta Journal-Constitution in 2009.
The congressional report describes how Deal and his chief of staff intervened with state officials in 2008 and 2009 on behalf of his auto salvage business, Recovery Services Inc.
The company's no-bid contract with the state earned nearly $1.5 million from 2004 through 2008. Graham wanted to open the inspection process up to introduce more competition.
Deal has since ended his relationship with the state.
The Washington-based watchdog Citizens for Responsibility and Ethics in Washington filed a complaint with the Office of Congressional Ethics, which effectively acts as a gatekeeper, conducting preliminary ethics investigations.
The office, in its report released Monday, recommended the Committee on Standards of Official Conduct launch an investigation of Deal's activities.
Melanie Sloan, CREW's executive director, praised the office for releasing its findings and suggested the Justice Department should pursue a criminal investigation.