Of all the Republican presidential hopefuls, none has been a quintessential Washington figure longer than Newt Gingrich.

After serving 20 years as a member of Congress, including two terms as House speaker, he did not go back home to Georgia, where he had been a history professor. Instead, he kept his hand in the political game, making lots of money in the process – much of it consulting for (or, as others suggest, lobbying on behalf of) major corporations.

In recent days, the list of Mr. Gingrich's big-name clients has continued to grow: mortgage giant Freddie Mac, Microsoft, General Electric, I.B.M., the US Chamber of Commerce, and the pharmaceutical trade group PhRMA (Pharmaceutical Research and Manufacturers of America).

As of August, reports the Daily Caller online news site, “Gingrich’s total assets were valued at between $6.7 million and $30.7 million in 2010. He has written 23 books and produced eight documentaries with his wife. He commands between $40,000 and $50,000 per speech.”

By comparison, Mitt Romney’s net worth is estimated to be between $190 million and $250 million held in blind trusts. But at the moment, at least, Gingrich’s wealth and business dealings have become more of a political issue.

Early in his presidential run, Gingrich was criticized for taking a two-week Greek cruise rather than spending time with potential voters – one reason much of his campaign staff resigned en masse in June. He also had to explain why he and his wife had a $500,000 line of credit at Tiffany’s.

Gingrich’s reasoning behind his financial success and personal wealth: “I've been a very hard-working business person.”

“I think I represent the wing of America that believes that hard work and success is good, not bad, and I'm happy to answer for it,” Gingrich told supporters in Jacksonville, Fla., this week.

Gingrich also has his own charitable organization, but that, too, has generated controversy.

“The charity, Renewing American Leadership, not only featured Gingrich on its website and in fundraising letters, it also paid $220,000 over two years to one of Gingrich's for-profit companies, Gingrich Communications,” ABC News reported. “It purchased cases of Gingrich's books and bought up copies of DVDs produced by another of the former House speaker's entities, Gingrich Productions.”

As a former history professor and prolific writer of books, Gingrich has had many opportunities to pen newspaper columns promoting his own ideas and political philosophy. But here, too, the work appears to have benefited him financially in more than one way.

“In a series of op-eds stretching over several years, Gingrich repeatedly advocated for various health-care related issues, including electronic health care records, ways to improve the health care sector, and medical malpractice reform without acknowledging the issues were directly connected to the members of the Center for Health Transformation, a for-profit think tank he founded in 2003,” reports USA Today.

Details of Gingrich’s dealings with the health-care industry continue to emerge.

Gingrich's health-care think tank has collected at least $37 million over the past eight years from major health-care companies and industry groups, offering special access to the former House speaker and other perks, according to records and interviews reported by the Washington Post.

“The Center for Health Transformation, which opened in 2003, brought in dues of as much as $200,000 per year from insurers and other health-care firms, offering some of them ‘access to Newt Gingrich’ and ‘direct Newt interaction,’ according to promotional materials,” reports the Post. “The biggest funders, including firms such as AstraZeneca, Blue Cross Blue Shield, and Novo Nordisk, were also eligible to receive discounts on ‘products and workshops’ from other Gingrich groups.”

Gingrich continues to insist that his various sources of income have been strictly legitimate, and that his many years since leaving office – the various ways in which he has strongly advocated for certain policy issues – have followed the letter and the spirit of laws regarding the ways of Washington business. He has never been a registered lobbyist, he points out.

"I will cheerfully answer every single question they ask,” he told the gathering in Florida. “And at the end of it, you will be relatively convinced, I believe, that I did no lobby of any kind, I did no influence-peddling of any kind.”