Hoping to stymie New Hampshire surge, has turned to two issues almost certain to move GOP primary voters: .
On both counts, Romney noted in recent Granite State campaign appearances, McCain has been crosswise with the majority of his own party.
But, in responding to the charges, McCain's campaign turned to off-the-shelf material sure to take some of the sting out of Romney's attacks - Romney's own words.
On these two hot topics and many others, past statements or positions by the former Massachusetts governor can be found that either completely contradict or at least dilute Romney's present day attacks.
That Romney has changed or at least modified his stance on a variety of issues is, of course, not new.
But as the primary campaign ramps up into its frantic final days before the first contests, and as Romney is forced to fend off a challenge from former Arkansas Gov.in Iowa and Arizona Sen. John McCain in New Hampshire, the problem is presenting itself anew.
Lacking a pure conservative record of his own, Romney is unable to get off any clean shots at his rivals without them - or the media - pointing to a past quote or stance that calls into question his own consistency.
Take last weekend in New Hampshire, when Romney took after McCain for opposing President Bush's tax cuts in 2001 and 2003.
The Associated Press account of the day included Romney's attack, McCain's counter and then additional evidence muddying the original charge.
In 2003, the story noted, Romney told the Massachusetts congressional delegation that when it came to the Bush tax cuts, he wouldn't "be a cheerleader" for proposals he didn't support.
"But I have to keep a solid relationship with the White House," Romney noted to his state's representatives in Washington.
Similarly, when Romney raised McCain's unpopular immigration views in a campaign appearance Wednesday, the Arizonan's campaign was ready.
"Last Year, Romney Supported 'Path Toward Citizenship' for Illegal Immigrants, Said Republicans Breaking With President Bush on Immigration 'Made a Big Mistake,'" McCain's aides reminded in a press release over 2006 stories in the Lowell Sun and Associated Press.
Also included was the November 2005 story from the Boston Globe where Romney deemed McCain's immigration approach "quite different" from amnesty and "reasonable."
Romney and his campaign have at-the-ready answers to counter the counters.
But his challenge is that there are seemingly few issues where he has not been previously more moderate than he is now or where a rival can't at least find a discrepancy sufficient to blur an attack.
Abortion is the one issue that he fesses up to having flat changed his mind on, but that the list only begins there.
On gay rights, campaign finance reform, gun control and even his own political identity, Romney has tonally, if not substantively, moved to the right.
The consistency question is one that Romney and his staff are sensitive about but also one that has effectively become baked into the campaign's narrative.
"I know that there are some - particularly in opposing campaigns - who will try to look at old quotes, and perhaps take them out of context and perhaps not, and go back 14 years or 15 years and say, 'You said this here, you said that there,'" Romney said in New Hampshire Wednesday.
"But ... if you want to know what I'd do as president, you can see what I did as governor."
Asked about this, Romney spokesman Matt Rhoades pointed to his candidate's statehouse record of "balancing budgets, creating jobs, enforcing immigration laws and reforming health care."
Rhoades didn't directly respond when aked how the campaign could take on rivals without its own impurities being brought up.
"On issues like immigration, Gov. Romney agrees with the majority of grass-roots Republicans," Rhoades said.
"The enforcement of immigration laws is an important issue, and Governor Romney's pro-enforcement policies are the right way to stop illegal immigration, while Senator McCain's advocacy of blanket legal status and benefits for those who break our immigration laws are exactly the wrong policies."
Politically, though, the changed positions pose a serious threat, especially in New Hampshire.
Huckabee lacks the resources or campaign structure to effectively strike back at Romney on his past stances.
And thanks to Romney's ads, mailers and automated calls - as well as Huckabee's current role as the front-runner - the focus of the race there has become more about the Arkansan's views.
But in New Hampshire, Romney's conversions are bound to matter more.
First, it's a matter of simple geography. Granite State voters, or at least the media outlets they're served by, have been following Romney's career since at least 2002, when he ran for governor, if not since 1994.
Familiarity may or may not breed contempt, but in Romney's case it has ensured tough coverage and deep skepticism.
First, there are the New Hampshire-penetrating Boston Globe and Boston Herald - from where much of the oppo springs.
Additionally, the local Boston TV affiliates - which also reach into the Granite State - include some aggressive, politics-focused reporters.
And perhaps most threatening of all for Romney, he has emerged as public enemy No. 1 of both the liberal-leaning Concord Monitor editorial page and the stalwart conservative Union Leader.
The Monitor printed a withering editorial Sunday explaining just why Romney should not be president, and, since getting behind McCain earlier this month, the Manchester-based Union Leader has kept up a steady drumbeat of editorials praising their pick and damning his top rival in the state.
The latter paper has become especially helpful to McCain in providing conservative validation and serving as an effective, independent tool with which to beat Romney over the head.
Which is exactly what the McCain campaign did when Romney launched his immigration shot Wednesday.
In addition to sending along a trove of their own research, McCain aides topped their release with a quote and link to a Union Leader editorial from just last week.
"If Republicans are voting for Mitt Romney because they think he would be tougher on illegal immigration than John McCain would be, they need to explain how Romney suddenly switched from supporting McCain's position just two years ago to attacking it (with distortions) this year," read the Dec. 19 broadside.
In addition to its allies in the press, McCain's camp retains firepower of its own.
Though considerably smaller than what he had before his summer implosion, McCain's staff still knows all of Romney's vulnerabilities and still has at easy reach a trove of oppo on him.
Still, McCain's task is not an easy one. Huckabee needs to hang on to an Iowa lead that seems increasingly precarious with each new news cycle dominated by his own vulnerabilities.
Though New Hampshire hasn't traditionally followed the course set by Iowa, a Romney victory there would instantly dispel the notion that he's slipping and provide an immense boost just four days before Granite Staters vote.
And then there is the Romney advantage that hangs over, not just New Hampshire, but the entire GOP contest: his personal wealth.
McCain and his staff can press the flip-flop narrative and have it underscored by the Union Leader and Monitor every other day.
But, thanks to his self-funding, Romney can attempt to drown out the naysaers by airing negative ads, dropping more tough mail and sending out robo-calls attacking McCain.
And, through these mediums, Romney's own past views don't get a mention.