Tonight's presidential debate will last 90 minutes. But for both President Obama and Mitt Romney, it's likely to come down to whether they can persuasively answer one question.

Workers prepare for the first presidential debate in Denver. (Ed Andrieski/AP)

For Obama, that question calls back to a projection his team released in December 2008, before he'd even been inaugurated president. It was part of a paper meant to sell the stimulus, and in it, the transition team predicted that if we passed the stimulus, unemployment would never rise above eight percent. Of course, unemployment was above eight percent before the stimulus even passed, and it's never, during Obama's entire time in office, been under eight percent. 

So expect Romney to ask some version of the following: You promised that if we passed your stimulus, unemployment would stay below eight percent. Instead, unemployment hasn't been beneath eight percent for even a single month of your administration. Isn't that, by your own definition, failure?

Obama's answer will go something like this: When that projection was made, preliminary data showed the economy was shrinking at an annualized rate of about 3.8 percent. The Federal Reserve and private forecasters thought the same. The real numbers, which we didn't know until years later, showed the economy shrinking at almost nine percent -- a Great Depression-like calamity. The fact that the recession was much worse than initially thought explains why the projections were overly optimistic. If the Obama administration hadn't done what it did, unemployment would be much higher today.

The problem for Obama's answer is that, even once we did know the recession was worse than initially thought, the White House's own budgets repeatedly projected a stronger recovery than we had. Obama can say, fairly, that they couldn't have seen Europe's implosion coming, or the Arab Spring and its hit to gas prices, or the tsunami in Japan, but at a certain point, this begins to sound like excuse-making. 

If Obama needs to explain why the economy isn't better today, Romney needs to explain why it would be better under his administration. The Obama team sees its opportunity in Romney's policies: If a tax cut for the rich didn't work when George W. Bush tried it, why will it work when Romney tries it? And how can he justify cutting health care for the poor when he's cutting taxes on the rich?

Romney's answer is that he won't cut taxes for the rich. His problem is that he hasn't proven it. He's released the details of his rate cuts, which amount to a $149,000 tax cut for the top one percent. He's released no details about how he'd pay for them, much less how he'd pay for them such that they don't raise taxes on the middle class. A number of independent studies have, in fact, concluded that doing so is mathematically impossible. 

We got a preview of Romney's answer to this question Tuesday, when he floated a new idea to cap deductions at $17,000. The only problem? He's released no details on how that would work. And most tax experts say that while it's an idea with merit, it wouldn't pay for Romney's cuts, and it's an approach that would end up raising taxes on the middle class to pay for the rate cuts for the rich.

But Romney's campaign has clearly been thinking about how to respond to this question. We'll see what they came up with tonight.