Rush Limbaugh kept up his relentless criticism of John McCain on Super Tuesday, opening his nationally syndicated radio program with a blast at McCain's conservative credentials and integrity.

Limbaugh, whose show is heard on 600 stations across the country, accused the McCain camp of being "dishonest," "insecure" and "resorting to the same kind of politics as Hillary Clinton."

The conservative talk show host was referring to a letter he received on Monday from Bob Dole, the Republican nominee in 1996, that was released by McCain's camp and became the root of a nasty back-and-forth between Mitt Romney and McCain and the candidates' surrogates.

Click here to read Bob Dole's letter.

Dole did not endorse McCain, but he described the Arizona Senator as a loyal Republican on many issues. "I cannot recall a single instance when he did not support the Party on critical votes," Dole wrote.

Limbaugh on Tuesday accused the McCain campaign of leaking the letter to the media and of "manipulating Bob Dole" by putting a "dishonest spin" on Dole's view of McCain.

Dole didn't scold me," said Limbaugh. "Not even Nixon pulled this kind of stunt."

The Dole letter generated more than talk show chatter on Super Tuesday. Romney had to backpedal after making what McCain and his supporters considered an affront to Dole.

Appearing on FOX and Friends, Romney said Dole "was the last person I would have wanted to write a letter for me."

"I think there are lot of people who think McCain's race is a bit like Dole's race -- that it's the guy who's next in line, the inevitable choice," said Romney, noting that Dole lost to Bill Clinton in 1996.

McCain shot back, saying in a statement: "Gov. Romney's attack on Bob Dole is disgraceful, and Gov. Romney should apologize. Bob Dole is a war hero who has spent his life in service to this nation and nobody has worked harder to build the Republican Party. Bob Dole deserves the respect of every American and certainly every Republican."

McCain told reporters on the plane en route to San Diego that "this isn't the way to end up this campaign, by attacking a genuine American war hero ... I don't know why he said it, but I hope that he will apologize and soon."

Former 2008 candidate Sen. Sam Brownback, who represents Dole's home state of Kansas, also released a statement calling for Romney to apologize.

Romney tried to call Dole, with no luck, from his plane as he criss-crossed the country in last-minute campaigning. But he said he has nothing to apologize for.

"Let me make it very clear. Senator Dole is an American hero, a war hero, a fine man and a great leader for our party," Romney said in Charleston, W.Va., where GOP voters picked former Arkansas Gov. Mike Huckabee in their state convention.

Romney said his comment about Dole was only meant to point out that "the selection of our nominee based on someone having served in the Senate a long time ... did not do well for us in that election."

He said he was referring to "that aspect," not Dole specifically, when he made his comments.

Limbaugh wasn't the only conservative talker to blast McCain over Dole's letter of support.

Laura Ingraham told listeners on her syndicated show that they should take Dole's opinion of McCain with a grain of salt.

Dole is a "four time loser" and neither McCain nor Dole are true conservatives, said Ingraham, who is also a FOX News contributor. "So when Bob Dole comes out and tells talk radio through a letter to Rush, that you should just back off . . . I think of 1996 and that horrific campaign run by Dole and his advisers," said Ingraham, who is backing Mitt Romney for the Republican nomination.

"A four time loser is going to lecture talk radio and tell them to be quiet? Are you kidding me. Is that really how you think we're going to move the conservative movement forward?" Ingraham said.

FOX News' Julie Kirtz, Shushannah Walshe, Carl Cameron and Malini Bawa and The Associated Press contributed to this report.