With poised to win the Republican nomination, Democrats are already gathering ammunition to use against him in the general election.
In more than a few instances, the best fodder has been provided by the candidate himself.
A case in point: As the economy was rising late last year as a major issue for voters, McCain in New Hampshire delivered this grenade, with its pin still in it: "The issue of economics is not something I've understood as well as I should," he said. "I've got Greenspan's book."
Those are not the only words that will come back to haunt him in November.
From the economy to Iraq to immigration to abortion, the Arizona senator's lengthy voting record and his primary season offerings to the Republican Party's conservative wing provide a deep vein for opposition researchers to mine for shifting positions and policy inconsistencies.
Democratic National Committee Chairman Howard Dean is already moving to redefine the presumed Republican nominee. In a fundraising appeal sent out Wednesday, Dean called McCain "a media darling" and warned that "from Iraq to health care, Social Security to special interest tax cuts to ethics, he's promising nothing more than a third Bush term."
The tough part for Democrats will be making any criticism stick. Republican rival Mitt Romney tried to no avail. The sharp, eleventh-hour assault launched by conservative talk show host Rush Limbaugh and a cadre of high-profile conservatives also failed to derail his candidacy.
Doug Schoen, a former adviser to President Clinton, says the Democrats must act quickly. "The trick is to get him on the flip-flops and not let him get back to the center where he can be a real force," he said.
The appeal of a flip-flop assault is that it could undermine McCain's reputation for taking tough stands and sticking with them no matter how the political wind blows.
Carter Eskew, a former adviser to Vice President Al Gore, puts it this way: "Go right after his strengths. Take the Straight Talk Express and push it off the rails."
Democrats are also convinced McCain is standing on soft ground on policy issues that are foremost in voters' minds. Tad Devine, a strategist to 2004 Democratic nominee John Kerry, says those substantive critiques should be the first line attack.
On the economy, McCain has tried to distance himself from his self-deprecating comment about his understanding (or lack thereof) of the economy. But his attempts have fallen flat in part because he's made the mistake of alluding to the weakness more than once.
In 2005, he sat down with Stephen Moore of the Wall Street Journal, and said: "I'm going to be honest; I know a lot less about economics than I do about military and foreign policy issues. I still need to be educated."
On the campaign trail, he's also suggested he'd look for a vice presidential running mate with strong economic credentials to balance weaknesses of his own. He tried to take that one back, too.
Those comments, coupled with McCain's relatively thin economic message, could leave him vulnerable to recasting by the opposition.
One broad theme that will be used against him is that he's offering little more than an extension of the Bush economic policies that have exacerbated the nation's wealth gap and brought about a return of giant deficits.
Democrats could also take some sharper shots at his economic plan, which centers on two core messages: cutting taxes and cutting spending.
On taxes, McCain's votes against President Bush's 2003 tax cuts and his explanation for them are likely to become major talking points. "I just thought it was too tilted to the wealthy and I still do," he said of those tax cuts. "I want to cut the taxes on the middle class."
Democrats are sure to argue that if the Bush tax cuts were "too tilted" toward the rich in 2003, they are only more so now.
McCain will have to square his previous comments with his call today to make Bush's tax cuts permanent and add new cuts for the middle class.
To recover the lost revenue from the tax cuts, McCain is promising to cut earmarks and wasteful spending -- a line that plays well with his party's fiscal conservative wing.
On this point, the senator is on firmer footing since he's earned solid credentials on the issue by leading some major fights against pork barrel projects.