Everyone is going to play numbers games to judge President Bush’s next economic policies. Top of the list will be Mr. Bush’s pledge to cut the budget deficit in half by 2009. Although this promise seems simple, it isn’t.

Let’s see. Chad Kolton, a spokesman for the Office of Management and Budget, says the pledge was made a year ago, when the projected deficit for 2004 was $521 billion, or 4.5% of gross domestic product.

Thus, the administration’s targets for 2009 are $260 billion, or 2.2% of GDP. But wait: The actual deficit for 2004 was $413 billion (3.6% of GDP). Should Mr. Bush be aiming at half of that? Then there’s Social Security. If Mr. Bush proposes borrowing to pay for “personal accounts,” will those amounts be added to the deficits? They should, but in Washington, who knows?

All this suggests much confusion and controversy. What’s the right target? Who says? Mr. Bush may claim he’s halving the deficit, while critics say he isn’t. But the convoluted arithmetic also holds a broader lesson about Mr. Bush’s second term. To succeed, Mr. Bush needs a strong economy.

Without it, the deficits will balloon as the government loses taxes and pays more in benefits. More important, without it, popular discontent – over jobs, wages, trade – could combine with opposition to other policies (on Iraq, terrorism, judicial nominations) to weaken Mr. Bush’s popularity. That would probably doom his ambitious legislative agenda, from Social Security to tax “reform” to immigration. The latest poll from the Pew Research Center shows Mr. Bush’s vulnerability: By a 50-45 margin, respondents disapproved of his handling of the economy.

Compared with the first term, you might rate Mr. Bush’s economic prospects favorably. Recall those first-term problems: a recession, the stock-market collapse, corporate scandals, and the terrorist attacks on September 11, 2001. The White House is now forecasting economic growth of 3.25% annually from 2005 through 2010 (on a comparable basis, growth in 2004 was about 4%).

Unemployment, 5.4% in December, will slowly drop to 5.1% by late 2006 and stay there. The administration’s predictions mirror many private forecasts.

“We’re shifting to growth of 3 to 3.5% a year,” says Nariman Behravesh of Global Insight.

Caveats? Well, yes. The forecasts don’t allow for the next recession – and recessions happen. Moreover, some economists dissent. Michael Evans, an independent economic consultant, thinks growth will average between 2% and 3%. “I expect stocks to be flat over the next two years,” he says.

There’s also a small minefield of specific threats:

Oil: Mr. Behravesh thinks prices will fall gradually to $37 from about $48 a barrel by early 2006. But any unexpected scarcities and higher prices would hurt. He figures that every $10-a-barrel increase shaves 0.5 percentage points off GDP growth.

The Dollar: Massive American trade deficits have caused it to depreciate by about 15% since early 2002 against major foreign currencies. Up to a point, that helps American exports; they become cheaper on global markets. The danger is that a continuing drop in the dollar could spill over into stock and bond markets. A falling dollar means foreigners’ investments in American stocks and bonds are worth less in their own currencies. They might stop buying American securities or sell. Stock prices could drop or even collapse.

Cheap Credit: It’s ending. Since last June, the Federal Reserve has raised its overnight Fed funds rate to 2.25% from 1%. More increases are expected. Although long-term rates on bonds and mortgages haven’t yet risen, many economists think they will. By late 2005, Mr. Behravesh sees rates on 30-year mortgages at 6.5%, up from today’s 5.75%. Home construction, housing prices and consumer spending could all weaken.

Greenspan’s Replacement: The Fed chairman’s term expires a year from now. No likely successor will instantly acquire his authority. Any mistakes could shake confidence.

If Mr. Bush dodges these and other dangers (a slowdown in China?), critics will still attack his budget deficits. In some ways, this is unfair. True, Mr. Bush doesn’t plan on ever proposing a balanced budget.

But most of his critics aren’t any better. (John Kerry also pledged to cut the deficit in half.) Despite Social Security “reform,” neither Mr. Bush nor Democrats are facing up to the spending explosion of the baby boom’s retirement costs. The reason: the biggest increases stem from health care (i.e., Medicare and Medicaid).

Still, Mr. Bush could pay an ironic price for his tolerance of sizable deficits. If the economy falters, it will be harder to apply the classic stimulus – cutting taxes or increasing spending. The already big deficits will act as a deterrent.