(CNN) -- GOP front-runner John McCain is cruising into Super Tuesday with a hefty lead in the polls, but he's drawing a backlash from some top conservatives who say he is too liberal to carry the Republican nomination.
Sen. John McCain has come under attack from conservatives who say is he too liberal.
Radio hosts Rush Limbaugh, Laura Ingraham, Hugh Hewitt and Lars Larson are among the conservative voices speaking out against the Arizona senator.
The attacks have gotten so heated that former Republican presidential nominee Bob Dole Monday wrote Limbaugh defending McCain.
CNN obtained the letter from a Republican source close to Dole.
In the letter, Dole says McCain is a "mainstream conservative" who supported the party on critical votes during Dole's time as the Senate Republican leader.
Dole says he remains neutral in the GOP contest and speaks kindly of all three remaining leading GOP candidates.
But the letter comes at a time Limbaugh is trying to rally grassroots conservative support against McCain.
"Whoever wins the Republican nomination will need your enthusiastic support," Dole says in the letter. "Two terms for the Clintons are enough."
Limbaugh has called McCain's rise the product of a "fractured" conservative base and an "uninspiring" GOP presidential field.
"McCain will kill conservatism as a dominant force in the Republican Party," Limbaugh said on his radio show Monday.
Meanwhile, conservative commentator Ann Coulter last week said she would support Democratic contender Sen. Hillary Clinton of New York over McCain.
"If he's our candidate, then Hillary is going to be our girl, because she's more conservative than he is," Coulter said on Fox's "Hannity and Colmes." "I think she would be stronger on the war on terrorism."
"John McCain is not only bad for Republicanism, which he definitely is -- he is bad for the country," she said.
Coulter took aim at McCain's positions -- particularly his fervent anti-torture stance -- and said he and Clinton differ little on the issues. Coulter also said she is prepared to campaign on Clinton's behalf should McCain win the party's nomination.
McCain has been at odds with some of the conservative base for his stance on immigration, his support of campaign finance reform legislation and his vote against President Bush's tax cuts.
Exit polls from the early voting states have shown that he has consistently lost among those who identify themselves as conservative. But he passed a key test last week in winning Florida's primary, the first early contest that allowed only registered Republicans to participate.
Rival Mitt Romney has been attacking McCain's conservative credentials as he tries to chip away from his lead. Watch what Romney says about McCain »
The former Massachusetts governor is telling voters he is the alternative to McCain, whom he compared to Clinton and Sen. Barack Obama of Illinois.
In last week's Republican debate, Romney criticized McCain for a recent endorsement, saying, "I'd also note that, if you get endorsed by the New York Times, you're probably not a conservative."
Romney predicted his party's conservative base would rally behind him Tuesday in order to keep the Arizona senator from taking the Republican nomination.
Former Sen. Rick Santorum, who endorsed Romney last week, recorded an automated phone call being used in several Super Tuesday states that directly questions whether McCain has the temperament to be president. Watch McCain's response to Santorum »
"As a conservative, I don't agree with McCain on many issues and I don't think he has the temperament and leadership ability to move the country in the right direction," Santorum says in the call, according to a Romney source.
Santorum notes that as a senator, he worked hard to stop Democrats and pass a conservative agenda, and "a few senators like John McCain stood in our way."
McCain, who went to Pennsylvania to campaign for Santorum in his ill-fated 2006 Senate re-election bid, said he was "puzzled" by the calls.
"He thought I had the temperament when he asked me to campaign for him and I did," McCain said Monday.
When asked why he thinks some conservatives are opposed to him, McCain replied, "I know these primaries are tough. I know people support other candidates. But look -- we wouldn't be winning these elections if I had any significant opposition anywhere in the party, but I'll unite the party, and I have a conservative record."
McCain continues to say he is proud of his conservative record, adding he has picked up support from some key conservatives.
Former New York Gov. George Pataki backed him Monday, and last week, California Gov. Arnold Schwarzenegger announced he was endorsing McCain.
McCain has also picked up the support of former Solicitor General Ted Olson, Georgia Sens. Johnny Isakson and Saxby Chambliss and Florida Gov. Charlie Crist. E-mail to a friend
CNN's John King, Alexander Mooney, Carol Costello and Dana Bash contributed to this report.