Mitt Romney boasts that he is “proud to be the only major candidate for president to sign the tax pledge. The others have not.”

In a radio ad airing in New Hampshire, Republican presidential candidate Mitt Romney boasts that he is “proud to be the only major candidate for president to sign the tax pledge. The others have not.”

But former Arkansas Gov. Mike Huckabee signed the same “no new taxes” pledge in March. So too have Sam Brownback, Duncan Hunter, Tom Tancredo and Ron Paul – all Republicans vying for president.

On a base level, then, Romney’s ad is incorrect.

But he did say he’s the “only major candidate.” And among the pledge signers, Romney is certainly the top candidate. So the ad’s truthfulness comes down to the definition of “major.”

Huckabee, who has been feeling momentum after standout debate performances and a second place finish in the Iowa straw poll, views himself as a major candidate (don’t they all?) and his campaign was quick to challenge the radio spot.

(A Washington Post-ABC News poll released in October 2007 showed Romney with 11 percent support nationally and Huckabee 8 percent, his best showing so far. But in state polls, Romney does far better and his fund-raising advantage is enormous.)

“There was no slight intended. The context was in line with comparisons between the top tier candidates in terms of fundraising and current poll numbers,” Romney spokeswoman Gail Gitcho said.

The Presidential Taxpayer Protection Pledge was started in 1987 by Americans For Tax Reform, a group headed by national conservative figure Grover Norquist. Every Republican candidate since 1988 has signed the pledge, according to the group.

But so far in the 2008 contest, other top GOP presidential contenders have not, including Rudy Giuliani, Fred Thompson and John McCain.

Romney has opposed a pledge in the past. In 2002, while running for governor of Massachusetts, he said he opposed all tax increases in principle but was not willing to put that in writing. His spokesman at the time called such pledges “government by gimmickry.”