Representative Ron Paul of Texas has emerged as the Republican presidential candidate most willing to attack his rivals with hard-hitting commercials.

Now, it is Newt Gingrich’s turn as Mr. Paul’s target.

In a new television ad showing in Iowa, Mr. Paul accuses Mr. Gingrich of “serial hypocrisy” for having promoted conservative principles as speaker of the House but later reversing himself while receiving money as a consultant.

The ad is designed “to debunk the myth that the Newt we are seeing on the 2012 campaign trail is the conservative he has been touted to be all along,” said Jesse Benton, Mr. Paul’s national campaign chairman.

Mr. Benton said the ad would run “heavily” on Fox News in Iowa. He declined to disclose how much money the campaign was spending to show it.

The ad highlights Mr. Gingrich’s work for Freddie Mac, the mortgage giant, and the millions his organization earned from health care companies even as Mr. Gingrich supported an individual health care mandate.

“Everything that Gingrich railed against when he was in the House, he went the other way when he got paid to go the other way,” the ad says.

Mr. Gingrich had been spared from Mr. Paul’s attacks a month ago, when the Texas congressman ran an ad critical of other Republican rivals, including Herman Cain, Rick Perry and Mitt Romney.

But Mr. Gingrich’s recent surge to the top of the polls in Iowa has revived attention on his record. So far, Mr. Romney and the other candidates have not taken direct aim at Mr. Gingrich, preferring to keep their attention focused on President Obama.

Mr. Paul, who is in second place in several Iowa polls, appears to have taken a different approach. While promoting Mr. Paul’s fiscal record, the campaign has not been shy about attacking his Republican rivals directly.

In another ad running in Iowa this week, he mocks his Republican rivals, saying that they are not “big dogs” when it comes to reducing the size of government. Rather, the ad says, they are “Shih Tzus.”