Somewhere between massive and apocalyptic, if an actual default were to ensue. That’s why I’ve never understood, throughout this whole endless tragicomic melodrama, how President Obama could possibly let that happen. It seems to me that definitive action — unilateral, if necessary — to prevent the nation from suffering obvious, imminent, grievous harm is one of the duties any president must perform. Perhaps the most important duty.

Neither Obama nor anyone else in the White House wants to talk about possible doomsday scenarios, except to warn that Social Security checks might not go out on time. This is understandable. There’s no incentive for Republicans to give an inch — which would anger the Tea Party base — if they believe Obama, in the end, will ensure they never have to face the consequences of their intransigence. In any event, there will probably be some kind of deal. Won’t there?

Time grows awfully short. It’s not clear, at this point, whether House Speaker John Boehner can even get a short-term “cut, cap and balance” bill passed. A hard-core Tea Party caucus of perhaps 30 members won’t vote for a debt ceiling hike under any circumstances., which means he needs Democratic votes. Such aisle-crossing usually comes after give-and-take negotiations, but Boehner says he won’t give.

Something will probably work out. But if it doesn’t, it seems to me that Obama has no choice but to act.

Many legal scholars believe Obama could cite the 14th Amendment and assert that it requires him to make sure the nation’s credit is never “questioned,” which would obviously be the case in a default. Another theory holds that the president has the implicit power to take extraordinary measures in the face of impending national crisis. This isn’t in the same league as Abraham Lincoln’s suspension of habeas corpus as the nation careened into civil war. But one could argue that the principle is the same.

Who knows how the courts — ultimately, the U.S. Supreme Court — would react? With outrage? With deference toward presidential power? With traditional reluctance to intervene in political disputes between the two elected branches of government? It would matter, eventually. But while legal briefs were being prepared and arguments honed, Obama would have raised the debt ceiling on his own authority — and the crisis would have been averted.

Of course, there would be immediate calls from Tea Party Republicans to open impeachment proceedings against the president. I have no idea how far they’d get; my hunch is, not very. Still, it would be one more thing — a big thing — for Obama to deal with as he fights for reelection. But the president may not have a choice. By any theory of the presidency, his responsibilities have to include stopping the country from hurling itself off a cliff.