After he left Congress, GOP presidential candidate Newt Gingrich made tens of millions of dollars from corporate clients, who paid his health-care think tank up to $200,000 a year for “access to top transformational leadership across industry and government.” Gingrich also bragged about his success in pushing conservative policies and legislation in Washington during his political exile.

Yet the former House speaker insists that he has never been a lobbyist.

“It’s written in our contracts that we do not do any lobbying of any kind,” the candidate told CNBC this week. “I offer strategic advice.”

Gingrich’s meteoric rise in Republican polls has reignited the long-standing debate in Washington over the definition of lobbying, which has a specific meaning under federal lobbying statutes that often is at odds with common sense.

Indeed, it is quite common to engage in “lobbying activities” under the law without having to register as a lobbyist.

“It is very easy to escape the need to register,” said Joseph Sandler of Sandler Reiff Young & Lamb, who co-chaired an American Bar Association’s task force on lobbying reforms. “There are thousands of people in Washington who lobby without being lobbyists.”

There are three main criteria that require a person to register under the Lobbying Disclosure Act, Sandler and other experts said. If any of the conditions aren’t met, the person doesn’t need to register. (The rules are slightly different for in-house lobbyists directly employed by corporations or trade groups.)

First, the person must have two or more contacts with a lawmaker, a congressional staff member or an administration official on behalf of a particular client. This doesn’t include routine scheduling or status communications.

Second, at least 20 percent of the time spent working for a particular client must consist of “lobbying activities.” Under federal law, that includes both direct contact with officials as well as “preparation or planning activities, research and other background work that is intended, at the time of its preparation, for use in contacts and coordination with the lobbying activities of others.”

Finally, the person must earn at least $3,000 from the client in a given quarter; the dollar amount has risen over time based on inflation.

This labyrinthine system means that many Washington power brokers, from former Senate majority leader Thomas A. Daschle (D-S.D.) to Gingrich, do not have to register as lobbyists despite their clear influence on the political process. Many former public figures, for example, work as strategists or consultants at firms that also do lobbying through other employees.

What’s more, the Justice Department has never prosecuted anyone for not registering as a lobbyist, although it has gone after a handful of firms for faulty paperwork.

The difficulty in defining lobbying has complicated efforts by reform groups and the Obama administration to limit lobbyists’ influence on the government. The White House came under fire early in President Obama’s term, for example, for banning the hiring of people who had registered as lobbyists within the previous two years, prompting objections from some human rights and good-government groups.

Obama has pledged not to accept campaign contributions from registered lobbyists. But Republicans accuse him of hypocrisy because some of his donors advocate on behalf of corporations or trade groups in Washington, even if the legal definition of lobbying doesn’t apply.

Lisa Gilbert, deputy director of Public Citizen’s Congress Watch program, said her group and others are working on proposals that would broaden the legal definition of lobbying with the aim of providing more information to the public about Washington’s influence industry.

“It’s safe to say that when someone like Newt Gingrich can claim not to be lobbying, the standard is too loose,” Gilbert said. “It’s the worst kind of revolving-door abuse. It’s clear that people are trading in on their contacts and their Rolodex from their time in office.”

In a report earlier this year, the bipartisan American Bar Association task force that Sandler co-chairs recommended more reporting requirements and restrictions for lobbyists, including limits on raising money for lawmakers whom they lobby. The group also suggested doing away with the 20 percent threshold to spur broader registration.

Even if such reforms were enacted, however, it’s not clear whether Gingrich’s work as a consultant would trigger the need to register as a lobbyist. The candidate and his representatives have made clear that he was careful to avoid being tagged as a lobbyist, a strategy that could still be pursued under tighter regulations.

At least one longtime K Street insider isn’t buying Gingrich’s defenses, however. Jack Abramoff, a disgraced Republican former lobbyist who served three years in prison for fraud, wrote in an essay for Reuters that “all of Washington lives in Newt’s swamp.”

“Many, like Gingrich, try to disguise their role in the lobbying monolith by declaring themselves consultants,” Abramoff wrote. “They cry that they are not ‘lobbyists,’ they are merely providers of strategic advice. I have news for them: that’s what lobbyists do!”