In a report released Monday, the Office of Congressional Ethics also found that Deal exceeded the limit on outside income he may earn as a member of the U.S. House.
The alleged violations may never be confirmed, however, because Deal resigned from 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.
Harris Blackwood, a spokesman for Deal, 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 in Georgia.
"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.
But Blackwood chalked that up to "an innocent accounting error" and said Deal had corrected the last 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 to change the way the state inspects rebuilt salvaged vehicles.
The report recommended that 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. 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," Cagle said in a statement Monday.
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. also known as Gainesville Salvage and Disposal.
The company's no-bid contract with the state earned nearly $1.5 million from 2004 through 2008. Graham was seeking to open the inspection process up to introduce more competition.
Deal has 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 that the congressional standards and conduct committee launch an investigation of Deal's activities. Deal resigned before the committee could move forward.
Melanie Sloan, CREW executive director, praised the office for releasing its findings and suggested the Justice Department should pursue a criminal probe.
"Mr. Deal shouldn't be able to dodge responsibility for his misconduct by hightailing it out of Congress before he could be held accountable," Sloan said in a statement.
Deal is one of seven Republicans running to replace Sonny Perdue, who is prevented by term limits from running again.