A combination of early presidential maneuvering and internal policy debate is feeding yet another iteration of that media perennial: the great Republican crackup. This time, it’s tea party insurgents versus get-along establishment fogies, fighting principally over two things: (a) national security and (b) Obamacare.

(a) National security

Gov. Chris Christie recently challenged Sen. Rand Paul over his opposition to the National Security Agency metadata program. Paul has also tangled with Sen. John McCain and other internationalists over drone warfare, democracy promotion and, more generally, intervention abroad.

So what else is new? The return of the most venerable strain of conservative foreign policy — isolationism — was utterly predictable. Republican isolationists dominated until Pearl Harbor and then acquiesced to an activist internationalism during the Cold War because of a fierce detestation of communism.

But now — after Afghanistan and Iraq, after drone wars and the National Security Agency revelations — the natural tension between isolationist and internationalist tendencies has resurfaced.

In fact, both parties are internally split on domestic surveillance, as reflected in the very close recent House vote on curbing the NSA. This is not civil war. It’s a healthy debate that helps recalibrate the delicate line between safety and security.

The more fundamental Republican divide is over foreign aid and other manifestations of our role as the world’s leading power. The Paulites, pining for the splendid isolation of the 19th century, want to leave the world alone on the assumption that it will then leave us alone.

Which rests on the further assumption that international stability — open sea lanes, free commerce, relative tranquility — come naturally. If only that were true. Unfortunately, stability comes about only by Great Power exertion.

In the 19th century, that meant the British navy, behind whose protection America thrived. Today, alas, Britannia rules no waves. World order is maintained by American power and American will. Take that away and you get chaos.

That’s the Christie/McCain position. Paul’s views are still in the minority among conservatives, but gathering strength. Which is why Christie’s stroke — defending and thus seizing the party’s more traditional internationalist consensus — was a signal moment in the run-up to the 2016 campaign.

(b) Obamacare

The other battle is about defunding Obamacare. Led by Sens. Mike Lee and Ted Cruz, the Republican insurgents are threatening to shut down the government Oct. 1 if the stopgap funding bill contains money for Obamacare.

This is nuts. The president will never sign a bill defunding the singular achievement of his presidency. Especially when he has control of the Senate. Especially when, though a narrow majority (51 percent) of Americans disapprove of Obamacare, only 36 percent favor repeal. President Obama so knows he’ll win any shutdown showdown that he’s practically goading the Republicans into trying.

Never make a threat on which you are not prepared to deliver. Every fiscal showdown has redounded against the Republicans. The first, in 1995, effectively marked the end of the Gingrich revolution. The latest, last December, led to a last-minute Republican cave that humiliated the party.

Those who fancy themselves tea party patriots fighting a sold-out establishment are demanding yet another cliff dive as a show of principle.

But there’s no principle at stake here. This is about tactics. If I thought this would work, I would support it. But I don’t fancy suicide. It has a tendency to be fatal.

The real question here is sanity. Nothing could better revive the fortunes of a fading Democratic administration than a government shutdown where the president is portrayed as standing up to Republicans on honoring our debts and paying our soldiers in the field.

How many times must we learn the lesson? You can’t govern from one house of Congress. You need to win back the Senate and then the presidency. Shutting down government is the worst way to get there.

Charles Krauthammer is a columnist for The Washington Post. He can be contacted at:

letters@charleskrauthammer.com