CONWAY, S.C. -- Sen. John McCain, stumping on behalf of Mitt Romney, appeared a bit rusty with his campaign hits this morning at a rally here. He tripped over an attack line meant to tie Newt Gingrich and Rick Santorum together and paint them as pork-barrel spenders.

"Lindsey Graham and Jim DeMint have joined with me time and time again go to the floor of the Senate to fight against the earmark, porkbarrel corruption that goes on in Washington, D.C. Earmarks are a gateway to corruption. I can tell you that neither Mitt Romney nor Rick Santorum share that view," McCain said, quickly catching himself, before continuing. "When Newt Gingrich was Speaker of the House, earmarks exploded. Rick Santorum sponsored earmark after earmark. I went down to the floor and fought against those."

"My friends, earmarks are the gateway to corruption. Members of Congress went to jail, and I guarantee you this president," McCain said, with a slight gesture towards Romney on his right, "will fight against it time after time."

The line, and a similar attack on Santorum the night before in Charleston, were notable for a simple reason: Romney campaign stops almost never include mentions of, let alone attacks on, his GOP rivals for the nomination, with the vast majority of time and energy spent targeting President Obama instead.

Even surrogate attacks on Romney's rivals are often more like air strikes than hand-to-hand combat. In December, former New Hampshire Gov. John H. Sununu labeled Gingrich "not reliable or trustworthy," on a conference call with reporters, and on MSNBC's The Daily Rundown said his "off-the-cuff thinking" was dangerous in a Commander-in-Chief.

Today, on MSNBC's Andrea Mitchell Reports, Sununu said Gingrich was "whining" and -- playing off Tom Hanks' character in "A League of Their Own" -- he added, "There's no whining in politics."

Surrogate attacks while the candidate stands on stage with the attacker, however, are extremely rare.

But today's attack on Santorum, who a new poll now shows climbing into second place behind Romney in South Carolina, was no maverick McCain move. Within hours, the campaign released a memo to remind reporters of McCain's successful attack on Santorum, delivered the night before in Charleston.

"Sen. Santorum and I have a strong disagreement," McCain said then, "a strong disagreement that he believed that pork barrel projects were good for America. I think it’s wrong for America."