Michael A. Cohen is a senior research fellow at the New America Foundation and the author of “Live From the Campaign Trail: The Greatest Presidential Campaign Speeches of the 20th Century and How They Shaped Modern America.” (Full biography.)

There is a popular view about presidential campaigns that image and perception, even likability are the crucial variables, with substance usually running a distant second. (The recent vice presidential debate is possibly the quintessential example as Sarah Palin’s connect-the-talking-points performance was considered by many pundits a success simply because the substantive expectations were so low.)

However, the second presidential debate in Nashville, Tenn., went a long way toward undermining this long-held perspective. Stylistically, Barack Obama certainly seemed more comfortable than John McCain, but last night it was on policy ideas and his vision for America where Mr. Obama truly shined, demonstrating in no uncertain terms why he is opening up a commanding lead in public opinion polls — and why John McCain has been unable to narrow the gap.

Throughout the debate, Mr. Obama maintained the substantive upper hand; constantly tying Mr. McCain to the toxic image of President Bush all the while offering a mix of policy initiatives from a middle-class tax cut and new investments in green technology to tax credits for small businesses that offer health care, increased oversight of Wall Street and new diplomatic overtures to Iran. Mr. Obama can still be characterized as light on specifics; and like many a change-oriented presidential candidate has a tendency to wallow in generalities. But these faults, notwithstanding, he is far better at providing a sense of his priorities and, most important, a vision for where he wants to take the country. For the first time in 18 months as a candidate, Mr. Obama truly cast a presidential air.

John McCain on the other hand fell back on many of the same proposals that have done little to arrest his steady decline; from attacks on earmark spending and fears of an Obama administration-led tax increase to calls for more offshore drilling and a fuzzy proposal for the Treasury Department to buy up bad mortgages. Again, few of them resonated, particularly since Mr. McCain seems incapable of relating them directly to the pressing concerns of voters.

No part of the debate better dramatized the divide between the two candidates than a question about the types of sacrifices that should be expected from the American people. Mr. McCain again dwelled on his familiar call for cuts in government spending and earmark reform. Considering the lovely paean to sacrifice that Mr. McCain offered in his acceptance speech at the Republican National Convention it seemed like a missed opportunity — Mr. Obama did not make the same mistake. He said:

Each and every one of us can start thinking about how can we save energy in our homes, in our buildings. And one of the things I want to do is make sure that we’re providing incentives so that you can buy a fuel efficient car … weatherize your home or make your business more fuel efficient. And that’s going to require effort from each and every one of us. I think the young people of America are especially interested in how they can serve, and that’s one of the reasons why I’m interested in doubling the Peace Corps, making sure that we are creating a volunteer corps all across this country that can be involved in the community, involved in military service, so that military families and our troops are not the only ones bearing the burden of renewing America. That’s something that all of us have to be involved with.

For some, such proposals may seem meager, but there was freshness to Mr. Obama’s answer that stood in stark contrast to Mr. McCain’s rather lifeless generalities. One gets the sense that during the last 18 months, Mr. Obama has thought long and hard about the direction he wants to take the country and the policy initiatives that would form the basis of an Obama administration. Mr. McCain has not; a view that I would imagine even the most hardcore G.O.P. partisans would have a hard time disputing.

Yet, none of this should seem terribly surprising. Throughout his political career, John McCain has never evinced much interest in economic policy issues — and it shows. In an election like 2000, where personality dominated, or 2004, where national security was the top issue, Mr. McCain would be a formidable candidate, but in today’s current economic environment he simply seems lost. Add in the electorate’s exhaustion, even loathing of President Bush and the challenge for Mr. McCain is that much greater.

For all of Mr. McCain’s efforts to talk about the negative potentialities of an Obama administration they ring hollow when compared to the reality of the present administration — and its potential continuation. Voters are far more inclined to embrace change when the status quo appears much worse and this is certainly the case in 2008.

But in some respects this is not completely Mr. McCain’s fault. He is the leader of a political party that has run out of steam. The Republican Party seems more and more like a spent and rudderless force, devoid of new ideas for how to govern the country, and wedded to its unbending political orthodoxies, of cutting government spending, removing regulation and reducing income taxes.

Indeed, more than one observer noted the contradiction of Ms. Palin on Thursday decrying overbearing government while in the next breath calling for more government oversight of Wall Street. On Tuesday, Mr. McCain had similar problems, as he called for a spending freeze right after pledging support for an expensive new program to buy up bad mortgages. With a potential $300 billion price tag, Mr. McCain offered little sense of how he would pay for the plan.

What has happened to the Republican Party is not unusual in American politics. A similar phenomenon befell Democrats in the 1970s and 1980s as they became so tied to their liberal orthodoxies they were unable to shift course and devise policies to respond to the changing needs and desires of middle class America.

Since the failure of the G.O.P.’s Contract with America in 1995-1996, the Republican Party has flirted with political decline; in 2000 a smart campaign by George W. Bush crafted around the notion of compassionate conservatism nabbed the White House. But since then the party has moved further and further to the right, out of the mainstream of American political thought, while at the same time Democrats, led by Mr. Obama, have reclaimed the political center with new ideas and a palpable energy for moving the country forward. The rising fortunes of the Democratic Party and the precipitous fall of the G.O.P. were on full display Tuesday night. At a time when John McCain needed desperately to find the masterstroke that would allow him to reclaim the initiative from Barack Obama, he found a G.O.P. policy cupboard bereft of ideas.

There are four weeks until Election Day, but the route for Mr. McCain’s political revival seems to be increasingly out of his hands. Perhaps the capture of Osama bin Laden or even a major terrorist attack could refocus the race on issues favoring Republicans, but barring such an unforeseen event, Mr. McCain will continue to sail against the political winds. Nothing that happened Tuesday night changed the fundamental trajectory of a race that now overwhelmingly favors Mr. Obama; indeed the policy divide between the two candidates offers compelling and unmistakable evidence as to why that is the case.