There are two ways to make the economic case against President Obama. These are not mutually exclusive, but in a debate with limited time or in a campaign with five weeks to go, you have to prioritize.

One approach is to compare what he promised to what he did. In a sleek, easy-to-view ad, American Crossroads takes that approach:

The other is to explain what Obama’s policies entail for the future — more debt, higher taxes and a lower standard of living.

The American Enterprise Institute study I referenced earlier, showing the tax burden on middle-class Americans that will be required to pay just the interest on the debt, would be one way of going about this. And Jim Pethokoukis of AEI explains that “a household making between $100,000 and $200,000 a year could find its tax liability higher by roughly $2,400 every year. Over ten years, that works out to $24,000.And when you add in the debt already accrued the past four years under President Obama . . . that’s another $1,600 a year. So now we are now talking about $4,000 a year, $40,000 over ten years.”

The Romney team is highlighting a similar study from economist Doug Holtz-Eakin, who concludes that to pay down the debt would require tax cuts on most everyone: “If spending plans are unchanged, it appears inevitable that the middle class will face higher taxes. Imposing the entire burden on millionaires ultimately requires tax rates exceeding 75 percent of their income, even if one-half of the adjustment occurs on the spending side. Under a similar assumption, and assuming that higher taxes are phased in slowly over the budget windows, it is reasonable to conclude that taxpayers making as little $30,000 will face higher taxes, and that the average increase for taxpayers would be roughly $1,500 annually over the next 10 years.”

Even with modest spending cuts (a “balanced approach”), Holtz-Eakin argues that many more but the “rich” will be paying off the debt left by Obama.

Of course, there is another way to whittle down the debt — increased growth, significant spending discipline and entitlement reform. But Obama isn’t suggesting proposals that would do any of that. (The $4 trillion debt-reduction plan is a sham.) We could also reduce the debt much more slowly, but it’s not clear that credit-rating companies or the bond market would tolerate that.

Given that the president has never put a serious entitlement, tax or spending plan on the table and never submitted a budget with less than a trillion dollars in deficit spending, the second, forward-looking approach seems preferable.

Romney is already running ads saying that we can’t take four more years like the last. But that doesn’t go far enough. If Obama keeps going along this route, we will face the fiscal crisis that Rep. Paul Ryan (R-Wis.) has been warning us about: much higher taxes and, very likely, another recession (which historically has been the case after a period of such low growth). In other words, things will get much worse if we don’t get off the road we are on.

That strikes me as an effective argument for voters looking for a reason to vote against Obama. Hey, you thought the last four years was bad? You ain’t seen nothing yet.