Mitt Romney has gone on the offensive in the wake of a stinging loss to Newt Gingrich in the South Carolina primary, citing the ethics investigation that tarnished Gingrich's waning days in Congress and calling on Gingrich to release documents that could show he engaged in wrongful activity while working as a consultant.

Romney sharply questioned whether Gingrich broke any rules while working with various industries after leaving Congress. He called on Gingrich to release a list of clients that retained his services while Congress was debating a new a prescription drug benefit for Medicare, suggesting that the former speaker was exerting influence over former congressional colleagues despite having not registered as a lobbyist.

Was he working or were his entities working with any health-care companies that could've benefited from that? Romney asked. That could represent not just evidence of lobbying but potentially wrongful activity of some kind.

New, Assertive Romney Tactics

While Romney did not provide any specifics, his aggressive tone suggested that the former Massachusetts governor is trying to paint Gingrich as a Washington insider who has disguised the extent to which he benefited from his connections in Congress. Gingrich has presented himself to voters as the outsider candidate, asserting that the Republican establishment sees him as a mortal threat.

We just need to understand what his activity's been over the last 15 years, and make sure that it's conformed with all the regulations that might exist, Romney said.

Florida was devastated by the collapse of the housing bubble, and Romney played on the state's continuing struggles to recover when he attacked Gingrich for reaping more than $1.6 million from working for Freddie Mac. He called on Gingrich to return the money, reprising a theme from a television attack ad the Romney campaign is airing in Florida.

While Florida families lost everything in the housing crisis, Newt Gingrich cashed in, the ad says. Gingrich resigned from Congress in disgrace and then cashed in as a D.C. insider.

Gingrich has repeatedly denied that he violated any laws after leaving Congress, and during a Monday morning interview on ABC's Good Morning America maintained that he did not do any lobbying work for Freddie Mac.

Romney has also called on Gingrich to release the findings from an ethics probe that led the House to reprimand Gingrich and fine him $300,000 for improperly financing two projects and misleading the ethics committee, the first time a speaker was convicted of ethics violations.

Of course he should, release the full findings of the investigation, Romney told reporters on Friday, adding that You know it's going to get out before the general election.