(CNN) - John McCain said his endorsement of Mitt Romney in the presidential race is not a response to Rick Santorum's recent surge in the Republican nomination process.
During an interview Thursday on CNN's "Starting Point," McCain said he "strongly disagreed" with the former Pennsylvania senator's support of earmarks when they served in the Senate together, but added "I still respect him."
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Santorum spoke out publicly against McCain during the last presidential race and they at times had differing opinions as members of the upper chamber in Congress.
The Arizona senator and 2008 GOP presidential nominee, who backed his former rival Romney Wednesday, also offered some advice to current contender Newt Gingrich.
He said calling an opponent a liar, as Gingrich did to Romney earlier in the week, crosses a line in politics.
"You need to be careful about being too angry," McCain told CNN's Soledad O'Brien. "The people of New Hampshire want to hear your views and vision for the future rather than attacks on other candidates."
McCain, who won the New Hampshire primary in 2000 and 2008, said he intends to help Team Romney however he can, although he acknowledged endorsements do not hold much weight.
"Endorsements have limited effect," McCain said. "The performance of the candidates is what makes voters decide."
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