ANALYSTS HAVE wondered at the Obama campaign’s flat-footed response to the candidate’s poor first-debate performance. It’s one thing to have an off night. It’s another to spend the next week whining about your opponent’s “lies” and wallowing in Big Bird trivia. How to explain an impressive political team performing so feebly?

Alas, the answer lies at the heart of President Obama’s reelection bid: the absence of a second-term agenda. Mr. Obama has yet to say how he would solve the immense problems awaiting the next president immediately after election. Until and unless he does, the only rationale he can offer voters is the urgency of stopping the other guys. As he told supporters in Los Angeles last week: “The last thing we can afford right now is four years of the very same policies that led us to this crisis in the first place. . . . That’s why I am running for a second term for president of the United States.”

We agree that Mitt Romney’s agenda, to the extent it’s been minimally sketched out, would send the nation in the wrong direction. The Republican says that he wants to close tax loopholes. But instead of using the savings to reduce the nation’s debt, as the Simpson-Bowles commission said is essential, he would pay for further reductions in tax rates.

Moreover, he refuses to say which loopholes he would close and on which taxpayers. Loopholes are unpopular in theory but popular in real life: We’re talking about tax breaks for mortgage interest, charitable giving, health insurance. So the likely outcome is not just that Mr. Romney would fail to reduce the debt but that he would widen it alarmingly.

What would Mr. Obama do? We know that he wants to create 100,000 jobs for math and science teachers. (There are about 6 million K-12 teachers in the country.) He wants to take away $4 billion in tax breaks from oil companies. (The deficit in the fiscal year that just ended was $1.1 trillion.)

And he wants to increase tax rates for the wealthy. That would put a serious dent in the deficit, but it wouldn’t solve the fiscal problem, which unaddressed will “lead to a level of federal debt that would be unsustainable from both a budgetary and an economic perspective,” the Congressional Budget Office recently said. A solution can come only from restraining spending, especially growth in Medicare and Medicaid, and raising revenue, and not just from the rich. Yet Mr. Obama has put forward no plan to curtail entitlement costs, while Mr. Romney at least is willing to say that benefits for wealthy Medicare recipients will have to be cut back.

Why the reticence? Maybe, after promising so explicitly four years ago that he would make hard choices and refrain from kicking the can down the road, he’s reluctant to make the same promises again. Maybe the usual Democratic “Mediscare” campaign is so useful he does not want to muddy the waters — or, more accurately, un-muddy them — with budgetary honesty. Either way, he’s left without much of a case to take to the people and, if reelected, without much of a mandate.

“Show me a policy,” Vice President Biden challenged Rep. Paul Ryan during their Thursday debate. “Show me a policy where you take responsibility.”

It’s not too late for Mr. Obama to listen to that advice.