Republican presidential candidates, former Massachusetts Governor Mitt Romney and former U.S. Speaker of the House Newt Gingrich, pose before the Republican Party presidential candidates debate in Sioux City, Iowa, Dec. 15, 2011. (JIM YOUNG/REUTERS)

MASON CITY, Iowa -- Newt Gingrich is getting asked more and more frequently on the campaign trail whether he lobbied Congress to support a Medicare prescription drug plan in 2003 -- and his campaign team smells a rat.

It turns out that several current and former congressmen supporting rival Mitt Romney have been saying that Gingrich personally lobbied them about the law, which Republicans supported reluctantly, contrary to his claim that he has never lobbied anyone on Capitol Hill. Gingrich said he never lobbied anyone but addressed the entire Republican caucus to tell them that he thought they should vote for the bill. He was not working on behalf of a client, so he was advocating, not lobbying, he said.

Gingrich spokesman R.C. Hammond accused the Romney campaign of coordinating the strategy to tar Gingrich with the “lobbyist” label, which has prompted reporters to pepper Gingrich with questions on the matter at numerous appearances across Iowa this week.

“That was a public position taken publicly,” Gingrich said after being asked, again, to address the issue at a campaign stop in the courtyard of a shopping mall here. “That was not lobbying by legal definition. You can be an advocate without being a lobbyist.”

Among those accusing Gingrich are U.S.Rep. Jeff Flake (R-Ariz.), Idaho Gov. Butch Otter, a former congressman, and New Hampshire state Sen. Jeb Bradley, also a former congressman. All three have endorsed Romney for president. At least one person who has not endorsed Romney, former representative Marilyn Musgrave of Colorado, also accused Gingrich of lobbying her.

Hammond said the accusation is particularly ridiculous because Gingrich was asked by the speaker at the time, Dennis Hastert, to address Republicans on the issue. Gingrich has addressed House Republicans regularly since leaving Congress more than a decade ago -- about once a year on average, he said.

Romney campaign spokesman Ryan Williams declined to comment on Gingrich’s charge.

This story has been updated.