GRINNELL, IOWA ? Political insiders almost always seize on the same subject and very often they are wrong about it, but it is hard to go (or call) anywhere in political America this month without hearing the same thing: Can you believe how far John McCain has fallen and how fast he’s done it?

How far? Far enough so that the onetime presumptive Republican nominee is, in at least one reputable poll, in third place, behind former Sen. Fred D. Thompson of Tennessee, who isn’t even running, at least not now. How fast? In the blink of an eye. For a swifter descent early in the political cycle, you would have to go back to George W. Romney, whose remark that he had been brainwashed on Vietnam sank an otherwise appealing 1968 candidacy.

McCain was at the top of the heap only months ago. Mitt Romney, who is George Romney’s son but more surefooted and with better hair, was then addressing questions that he was a flip-flopper. Former New York Mayor Rudolph Giuliani, who is going to appeal more to religious conservatives than the experts think, was then a curiosity, nobody’s serious candidate, not even his own. Mr. Thompson wasn’t on anyone’s radar screen, which is an image you can use when you’re talking about an actor who appeared in “The Hunt for Red October.”

So what gives? What explains McCain drawing less than half the support of Giuliani in a Zogby International poll of likely caucus-goers in Nevada, which is next door to McCain’s Arizona? Surely it is more than McCain being brainwashed in an Iraq bazaar. No one knows the answer, but no one doubts the danger he’s in.

Mr. McCain’s problem may be that he advanced and flourished as a symbol and now is being crushed as a symbol.

In the old days, he was a symbol of American resolve, strength, courage, character, ingenuity and resourcefulness. He survived the Hanoi Hilton, told the truth to voters, took on the panjandrums of his own party. People liked that.

Now he’s a symbol of something else entirely: the mess in Iraq, the bungled military response to the crisis, the increasingly hopeless outlook there. “Republicans are in a funk, and Iraq reminds them of why,” says Michael W. Mahaffey, the Poweshiek County attorney and a onetime Iowa Republican chairman. McCain is collateral damage. Every roadside bomb in Iraq hurts his campaign in Iowa.

Mr. McCain has good people on the ground in Iowa, some with ties to the religious conservatives who are so important in this state, many with ties to veterans groups, which is important not only because McCain is the veterans’ veteran but also because the thing veterans do best (and other voters do very rarely) is to go to meetings, which essentially is what an Iowa precinct caucus is.

No one here or elsewhere thinks that McCain is out of the GOP race. He’s come back from too much, with too much strength, for anyone to say that. But the question that faces him is ironic, even Shakespearean: Will the political figure who defined himself eight years ago as being the politician who dared take on George W. Bush in the Republican primaries and caucuses be denied his own chance at the GOP nomination because he is too much the president’s man in 2008?

“Do people give him credit for sticking with the president in Iraq, or do their concerns and reservations about the war make them turn from him?” asks Mr. Mahaffey, who is not affiliated with any of the Republican contenders. “This is more of a political burden for John McCain than for anyone else.”

Many of the indicators are bad. Mr. McCain is the oldest presidential candidate in either party. He spent more than two-thirds of the $12.5 million he raised in the first quarter of 2007, leaving him with less than half of the cash that Giuliani and Romney have at their disposal. In an interview published last week in The New York Times, McCain said of Iraq: “I have no Plan B.” Richard Nixon may have had no Plan B for Vietnam but he had the virtue (though not the virtue of truthfulness) of saying that he had a secret plan.

Mr. McCain is not Richard Nixon, but he has one of the burdens Nixon had in 1968, which was to prove his conservative bona fides in a party that four years earlier had nominated Barry Goldwater. At this month’s Lincoln Day Dinner in Des Moines, former Gov. Mike Huckabee of Arkansas made a telling remark: “Nobody but a conservative has a hope of being elected president from our party.”

It is true that McCain is more conservative than many Democrats thought he was, wooed as they were by the McCain-Feingold campaign finance legislation and wowed by Mr. McCain’s habit of speaking truth to power, especially Mr. Bush. But is he conservative enough? Proving that he is, and retaining the free-speaking, free-wheeling, free-dealing independence that marked him in the House and the Senate, is going to be no easy task.

Mr. McCain has traveled to Iraq five times. When he returned from his latest visit, he went to Lexington, Va., to the fortress-like grounds of the Virginia Military Institute, and set forth two questions, for his party and for his country:

“Will this nation’s elected leaders make the politically hard but strategically vital decision to … do what is necessary to succeed in Iraq? Or will we decide to take advantage of the public’s frustration, accept defeat and hope that whatever the cost to our security, the politics of defeat will work out better for us than our opponents?”

These are important questions for the Republicans and for all Americans. Much depends on them. And one of the things that will be answered when the nation addresses those questions is the political destiny of John McCain, Republican of Arizona, once the front-runner for the Republican presidential nomination.