Among Republicans, conservatives, and some media commentators, we're seeing a backlash develop against Senator Obama. The form it takes is mocking the “cult of personality” we are seeing arise around him and insisting that his rhetoric, while uplifting, is essentially content-free. Obama, the argument goes, is an empty vessel in whom people are investing their own hopes and wishes. Hence his popularity.

There is something to this critique. His speeches, particularly of late, have been largely free of substance; he doesn't explain in any concrete way what change he hopes to bring about should he be elected President. If you compare Obama's rhetoric with, say, Martin Luther King, Jr., you'll see how glaring the difference is. King used his rhetoric — at once beautiful, uplifting, and compelling — to advance a philosophy and a cause: the belief that America needed to make good on its “promisary note” and do away with racial injustice. Like Lincoln, King grounded his arguments in an appeal to the Declaration of Independence and the moral law and had concrete implications. Obama's rhetoric, which can also be inspiring, seems almost consciously bereft of unifying ideas or political philosophy.

At the same time, Obama has given substantive policy speeches — and in debates and interviews he is perfectly capable of laying out his ideas. He is obviously conversant on issues like health care and many others. For conservatives to criticize Obama for his rhetorical vacuousness and make fun of the rock-star response he elicits in voters will, I think, ultimately prove ineffective. It's worth remembering how vague and airy John F. Kennedy's invocation of the New Frontier was during the 1960 campaign.

The area where Obama is vulnerable is his record, as brief as it is, and his stated positions. Senator Obama is a completely orthodox liberal — the most liberal person in the Senate in 2007, according to National Journal — in a nation that is not. Why hasn't this fact hurt Obama so far? Because his two main opponents in the Democratic race, Hillary Clinton and John Edwards, haven't advertised Obama's liberalism. They are essentially as liberal as Obama is, so that political arrow has been removed from their quiver. Hillary Clinton has therefore been forced to criticize Obama for his lack of experience — even though her own experience is quite thin and her past forays into policy have been disastrous (her mishandled health care plan helped the GOP win 52 House seats in 1994 and gave them control of the House for the first time in four decades).

When you analyze what her much-hyped “35 years of experience” actually involves — 20 of those years Mrs. Clinton was first lady, first of Arkansas and then of America — one is struck by how weak her claim is that she would be “ready on day one.” The dirty little secret is that Hillary Clinton is a mediocre-to-average candidate with little governing experience; she is where she is mostly because of the achievements of her husband.

Senator McCain's line of attack against Senator Obama, should Obama become the Democratic nominee, ought to be on Obama's liberal stands. As best as I can ascertain, apart from calling for merit pay for teachers, Obama is a conventional liberal on every significant national issue. Senator McCain needs to focus like a laser beam on that fact.

I should add an important caveat: invoking the liberal label is not enough. Especially against Obama — who is skilled, dexterous, and projects a sense of being non-ideological — much more will be necessary. Senator McCain needs to make deep, sustained arguments on behalf of liberty, limited government, constitutionalism, the family, and American strength and military power (from prosecuting the Iraq war to a successful conclusion to effective terrorist surveillance policies) in confronting militant Islam. He then needs to lay out a robust governing agenda based on those governing principles. And he needs to present himself as a reformer of our institutions, which is something that does come easily to McCain.

Making the case against Obama's liberalism will bring howls of protest from reporters and columnists who once held McCain up as a courageous “maverick” and who took particular delight when he antagonized conservatives. John McCain's days as the mainstream media's favorite Republican are about to end. One can already anticipate the avalanche of columns denouncing McCain as a flip-flopping, unprincipled panderer.

The mainstream media will insist that using the liberal label is so 1980s. Such name-calling, we will be told, is anachronistic, “old and tired,” simple-minded, and a sign of desperation. It may have worked against Michael Dukakis in 1988, they will argue, but we are a better and wiser nation now.

McCain should reject such counsel.

If Obama should win the Democratic nomination, John McCain should go straight at his record — with precision, without rancor, and relentlessly. John McCain will not win a personality contest against Barack Obama; no political figure in America could. McCain will have to base his campaign on a set of creative, forward-looking ideas that meet the challenges of our time. He will have to make this a race about ideas and about ideology.

It may not work; it will require McCain to do what he has shown no real facility for doing in the past. But it is his only chance. The best thing working in McCain's favor is that America rarely elects a president who is a thorough-going liberal — and Barack Obama is that if he is anything at all.

— Peter Wehner, former deputy assistant to President Bush, is a senior fellow at the Ethics and Public Policy Center.