A group of Republican lawmakers is threatening to use the potential government shutdown to defund Obamacare. Sen. Marco Rubio (Fla.) is among those leading the charge as more than 60 House Republicans and a dozen GOP senators have pledged to use the leverage of a shutdown to stare Obamacare to death. As Vice President Biden would say, God love ’em. But it isn’t going to happen.

John Boehner House Speaker John Boehner (Brendan Smialowskia/AFP/Getty Images)

Meanwhile, the president is threatening to shut down the government unless the sequester is replaced in part with tax hikes. That isn’t going to happen, either.

Let’s take the Republicans first. I think we’ve been through this a few times. Threatening to shut down the government (not to mention threatening to bump up against the debt limit) is not something Republicans have ever found palatable, and wisely so. The public doesn’t like it. The GOP will get blamed. And in the end, it will have to relent because this president is never, ever going to allow Obamacare to be defunded. It is an empty threat.

In a conference call with conservative journalists this morning, House Majority Leader Eric Cantor (Va.) declined to either endorse or rule out a shutdown. Instead, he touted the bipartisan votes on delaying the employer and individual mandates.

Moreover, the conservative American Action Forum explains via a mass e-mailing:

On September 30, funding for the government runs out. Congress could continue the current level of funding (a so-called continuing resolution) for some or all of the next fiscal year.  And it could cut out the dollars used by the Department of Health and Human Services (HHS) for the salaries of those setting up ObamaCare, the grants to state to set up ObamaCare, and so forth. In this sense, it could “defund” ObamaCare and impede its startup.  But the real money — trillions of dollars — is in the subsidies to individuals to buy insurance.  This is mandatory spending that will occur unless the ObamaCare law itself is modified. In this sense, it is quite difficult to “defund” ObamaCare.

The politics of defunding ObamaCare also appear less than straightforward. Democrats, especially the president, are unlikely to agree to defunding ObamaCare, leading to a government shutdown. ObamaCare is unpopular, indeed. But the only thing less popular would likely be a full-scale stop in federal services.

Precisely so.

Then there is the president. He couldn’t get rid of the sequester before it happened, and now that it is in place, helping to hold down the lid on the deficit, it is even harder to make the case that it should go. Frankly, it’s not an option I like because the implications of defense cuts are dire. But I can see the GOP isn’t going to budge.

Listen, the ground rules for Obama’s second term were pretty much set earlier this year. Whatever is already law (e.g. the sequester) can’t be stopped; whatever isn’t law (e.g. tax hikes) isn’t going to get done. That is the result of divided government.

It is swell for the GOP to promise to defund Obamacare. But when that falls by the wayside, it better have a backup plan.