Americans for Tax Reform, a conservative taxpayer group, regularly asks Republican politicians to sign a pledge not to raise taxes. Three Republican presidential candidates have not signed the pledge, which one strategist said might hurt them during the primaries.

"I worked on Bob Dole's campaign in 1988 and he didn't sign and it killed his campaign in the final week," David Johnson, a Republican strategist and president of Strategic Vision. "That's how the first President Bush was able to turn around and win the New Hampshire primary."

New York Mayor Rudy Giuliani, former Tennessee Sen. Fred Thompson and Sen. John McCain (R-Ariz.) have not signed the pledge.

"It carries a lot of weight in New Hampshire and it's something that can be used very effectively against a candidate, especially if it's a tight race as we're seeing in the polling right now," said Johnson.[

"The reality is nobody has to worry about whether I'm going to lower taxes, because I did it in a place that was harder to do it than in Washington," said Giuliani on Fox News in October.

"All the rest of them have never done it. So they got to take pledges. I have a record. In fact, that's sort of my distinction with most of the rest of the candidates. I actually have results," he added.