(CNN) – Sen. John McCain knows his way around rough politics: an opponent started rumors during the 2000 presidential race that he fathered an illegitimate African-American, and a CNN fact check called false his 2008 vice presidential pick's assertion that now-President Barack Obama was "palling around with terrorists."
But the veteran politician from Arizona – first elected to the Senate in 1986 – said Sunday that the current Republican presidential campaign is the most brutal race he has seen, fueled in part by super PACs that have rivaled campaigns in spending on this election.
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"This is the nastiest I have ever seen," McCain said on NBC's "Meet the Press."
The two-time presidential contender is not watching this GOP primary entirely from the sidelines: McCain is among several high-profile political figures actively stumping for Mitt Romney, and one of his roles has been to attack former House Speaker Newt Gingrich on behalf of the former Massachusetts governor.
"When you have a Las Vegas casino mogul - by the way, who gets part of his money from Macao - pouring $20 million into a campaign, obviously that drives up people's unfavorables," McCain said of donations to a pro-Gingrich group by casino magnate Sheldon Adelson and his wife. A casino in the Chinese semi-autonomous region of Macao is owned by a company under Adelson's control. Forbes lists him as the seventh richest person in the United States.
Another super PAC favoring Gingrich, Winning Our Future, spent $1.3 million on advertising, most of it critical of Romney, in South Carolina, according to CNN media consultant CMAG. A source close to Adelson told CNN in January that the donations were with "no instruction" and that Adelson had no role in strategy or advertisement content.
McCain, co-author of campaign finance legislation known as "McCain-Feingold" that limited corporate and private donations, has been harshly critical of the U.S. Supreme Court's "Citizens United" decision that cleared the way for super PAC spending. He predicted Sunday "there will be scandals and then maybe we will reform again."
The super PAC funding, as well as weakness on Romney's part, is keeping Gingrich's campaign afloat, McCain said.
"Mitt Romney will tell you he has to do a better job and he has to focus on the economy. And he has been giving speeches on the economy and jobs, and I think he is improving dramatically as a candidate," he added.
– CNN Senior Producer Kevin Bohn contributed to this report.