It's a sensitive question for McCain in South Carolina, where his presidential campaign in 2000 was famously derailed by an onslaught of negative attacks.
On Monday, after his campaign in that state sent out a mailer blasting Romney's record on taxes and accusing him of supporting taxpayer funded abortions, McCain faced a flurry of questions about the brochure from campaign reporters.
Who struck first? Romney's campaign, which originally circulated the mail piece to reporters on Saturday, points the finger at team McCain, calling the mailer a "hit piece."
McCain, campaigning in Michigan, told CNN he was playing defense against prior Romney attacks.
While he did not speak to which specific charge from Romney he was responding, McCain said his campaign's mailer was a justified strike against the "stack of mailers this high that attack me, literally this high that have flooded South Carolina for the last six months."
(Romney's campaign has sent out several pieces of issue-based contrast mail in South Carolina, but to this point, few, if any, mention McCain by name, and most target Mike Huckabee.)
But as it turns out, Romney's campaign did, in fact, send out a mailer criticizing McCain on taxes almost two weeks ago, which bolsters McCain's claim to playing defense, although it went unmentioned by the senator on Monday.
That Romney brochure, provided to CNN by a McCain aide, is a piece on contrast mail that asks, "Where do the candidates stand on fiscal responsibility?" and criticizes the tax records of McCain, Huckabee and Rudy Giuliani.
The mailer, which resembles similar pieces previously sent out by the Romney campaign, features a two-sentence mention of McCain's early opposition to the Bush tax cuts, and also criticizes two other Republicans.
By contrast, McCain's controversial mailer focuses its heat exclusively on Romney, hammering the former Massachusetts governor from four different angles on taxes, charges the Romney campaign calls "unsubstantiated" and "sloppy."
McCain told CNN Monday that his mail is "not negative campaigning" and he reserved the right to " respond in a very forceful fashion" to Romney.
Romney senior adviser Warren Tompkins, the hard-nosed South Carolina political consultant who worked against McCain on behalf of George W. Bush in 2000, accused the Arizona senator of being duplicitous.
"This isn't the first time that John McCain has tried to hang on to the high moral ground with one hand while slinging mud with the other," Tompkins said. "It's vintage angry McCain, and it's self-destructive because voters see right through it."
- CNN South Carolina Producer Peter Hamby