And we’re not talking about government programs nobody cares about: the only way to cut spending enough to pay for the Bush tax cuts in the long run would be to dismantle large parts of Social Security and Medicare.

So the potential cost of giving in to Republican demands is high. What about the costs of letting the tax cuts expire? To be sure, letting taxes rise in a depressed economy would do damage  but not as much as many people seem to think.

A few months ago, the Congressional Budget Office released a report on the impact of various tax options. A two-year extension of the Bush tax cuts, it estimated, would lower the unemployment rate next year by between 0.1 and 0.3 percentage points compared with what it would be if the tax cuts were allowed to expire; the effect would be about twice as large in 2012. Those are significant numbers, but not huge  certainly not enough to justify the apocalyptic rhetoric one often hears about what will happen if the tax cuts are allowed to end on schedule.

Oh, and what about confidence? I’ve been skeptical about claims that budget deficits hurt the economy even in the short run, because they undermine confidence in the government’s long-run solvency. Advanced countries, I’ve argued, have a lot of fiscal leeway. But anything that makes permanent extension of obviously irresponsible tax cuts more likely also sends a strong signal to investors: it says, “Hey, we aren’t really an advanced country; we’re a banana republic!” And that can’t be good for the economy.

Last but not least: if Democrats give in to the blackmailers now, they’ll just face more demands in the future. As long as Republicans believe that Mr. Obama will do anything to avoid short-term pain, they’ll have every incentive to keep taking hostages. If the president will endanger America’s fiscal future to avoid a tax increase, what will he give to avoid a government shutdown?

So Mr. Obama should draw a line in the sand, right here, right now. If Republicans hold out, and taxes go up, he should tell the nation the truth, and denounce the blackmail attempt for what it is.

Yes, letting taxes go up would be politically risky. But giving in would be risky, too  especially for a president whom voters are starting to write off as a man too timid to take a stand. Now is the time for him to prove them wrong.