Health and Human Services made it clear a shutdown won't stop Obamacare's launch. | REUTERS Obamacare exchanges still set

It’s looking more and more like Tuesday will be a split-screen day: The government will shut down, and Obamacare will open for business.

That’s going to annoy a lot of Republicans because the ones who are pushing the shutdown are doing so precisely because they want to halt Obamacare.


Barring some last-minute deal, which is getting less likely with each passing moment, the federal government will shut down on Tuesday because the House and the Senate are unable to pass a spending bill that resolves their deep and persistent differences over Obamacare.

( Also on POLITICO: Obama keeps distance from Hill)

But Tuesday is also the day Obamacare’s new health insurance marketplaces — some run by the states, many run by the feds — are scheduled to start signing up customers. And President Barack Obama has made clear that even if the government closes, the health care show will go on.

“On Tuesday, about 40 million more Americans will be able to finally buy quality, affordable health care, just like anybody else,” Obama said on Friday. The new health insurance exchanges, he said, “will be open for business on Tuesday no matter what — even if there’s a government shutdown. That’s a done deal.”

How is that possible? Here’s a guide to how it will work — and what we might see on Tuesday:

How can Obamacare still be open for business if the government shuts down?

Many pieces of the health care law, the Affordable Care Act, aren’t tied to the annual spending bills. Much of the health law is mandatory spending — a kind of fiscal autopilot that’s not part of the annual appropriations battle that has Congress tied in knots. The mandatory components of the health law include the subsidies to help people buy private health plans as well as the expansion of Medicaid in many states. Both of those functions will be handled through the new health insurance markets or exchanges.

( Also on POLITICO: Hill prediction: Headed for shutdown)

Because those programs are mandatory, the Department of Health and Human Services has a lot of leeway to say whether Obamacare activities can continue — and HHS officials have made clear they’re going to use it.

On Friday, the HHS quietly posted its shutdown contingency plan. The bottom line is clear: Obamacare would continue, including the health exchanges and their coordination with Medicaid. It also said Medicare coverage “will continue largely without disruption.” True, lots of HHS workers would be furloughed — but those who would be told to stay home are concentrated in agencies that are not driving the launch of the health law.

HHS says its plan is consistent with legal advice that allows activities that “do not rely on annual appropriations, and activities that involve the safety of human life and protection of property” to keep running even if much of the government shuts down. And that means the staff that carry out mandatory programs like those in the health law can keep working — even if their positions are funded through the annual spending bills.

( WATCH: Mitt Romney thinks there's a better way to stop Obamacare)

There’s plenty of legal precedent. In a report to Sen. Tom Coburn (R-Okla.), the Congressional Research Service predicted that “substantial ACA implementation might continue” during a government shutdown, citing a 1981 opinion by then-Attorney General Benjamin Civiletti. He argued, in essence, that as mandatory programs march on, they can take some discretionary-funded staff with them.

In addition, nobody really expected a huge rush to sign up on Day One. Even before the shutdown seemed so likely, the administration has been saying it expected people to spend some time learning more about their new health care options before committing to a health plan. Enrollment runs from October through March. Benefits kick in Jan. 1.

But at a time when so many Americans still don’t understand the law — and some don’t even realize that it hasn’t been repealed by the House or tossed out by the courts — the shutdown could add yet another layer of confusion. People may not realize they can sign up even if the government is shut down.

( WATCH: Obama to Congress: 'Pay our bills on time')

Where will they get the money to keep the exchanges open?

The exchanges have two main monetary needs: staff and infrastructure to keep them up and running, and insurance subsidies that will be available to people with low and moderate incomes.

The subsidy question is fairly simple. Like Medicare and Medicaid, the subsidies are benefit payments that will go out even if the government shuts down.

And for the exchange operations, HHS has already granted billions of dollars to the states setting up their own — 16 states plus the District of Columbia.

If the shutdown lasts for only a few days or weeks — the most likely scenario — the state-run exchanges should still be going full steam ahead, equipped with the funding and staffers they’ll need to start enrolling people right away. Some state exchange directors last week voiced concerns about whether they’ll get everything they need — including data on income and eligibility — from the feds. But they said they are ready to go.

The 34 exchanges, being run all or in part by the federal government, will also be up and running Oct. 1, although only mandatory funding or funds already appropriated through the ACA can be used to keep them staffed and running. Such money is available to several HHS agencies, including the Centers for Medicare & Medicaid Services, which is in charge of implementing Obamacare.

“Several HHS agencies have substantial mandatory, carryover or user fee funds which are not affected by a hiatus in annual appropriations, with CMS having the most mandatory funds,” the HHS shutdown contingency plan states.

Will everything run smoothly?

Not likely. The exchanges are already facing other delays and glitches even without the complications of a government shutdown. How quickly officials can troubleshoot and fix the widely expected snags and glitches in a shutdown scenario isn’t totally clear.

Just last week, HHS announced that the small-business health exchanges run by the feds will have to delay online enrollment until November. And the District of Columbia health exchange announced that it won’t be able to calculate people’s eligibility for subsidies and Medicaid online right away because of a “high error rate” discovered during the testing.

There have been reports of other problems, as well — The Wall Street Journal reported that the software used to calculate the subsidized prices of health coverage in the federal exchanges wasn’t working right as recently as two weeks ago.

That doesn’t mean they won’t be able to open and start signing people up — and the coverage won’t start until Jan. 1, anyway. But it was never going to be a smooth ride, and the strain on HHS — with those widespread furloughs in other agencies — could still affect the exchanges in ways that won’t be clear until Tuesday.

What will we see?

People who are not deterred by the shutdown and want to sign up for coverage online will see big differences in the website features of the states that are running the exchanges on their own. California, for instance, will look a bit different from Maryland.

But in the 34 states where the exchanges are totally or partly run by the federal government, the websites are likely to look more similar. Each state will have some of its own basic branding, but there will be a standard design for the rest, according to experts who have been consulting with state and federal officials.

But in each one, consumers will be able to compare the prices of health plans with different levels of coverage — bronze, silver, gold and, in some cases, platinum. They should also be able to find out whether they qualify for the tax credits that can help bring down the so-called sticker price. Those will be available on a sliding scale for people who earn up to 400 percent of the federal poverty level — currently $45,960 for an individual and $92,200 for a family of four.

And ultimately, they should be able to enroll online. The key question, though, is whether all of those features will work or whether websites will crash or give incorrect answers. And given the track record so far, there are sure to be problems — but whether they’re trivial won’t be known in full until Tuesday.

How long can the exchanges stay open in a shutdown?

The longest U.S. government shutdown lasted around two weeks — so based on history, a shutdown isn’t likely to last long.

But even if a shutdown does last for months, the exchanges will probably remain up and running — although the federally run exchanges could suffer under staff furloughs.

It seems clear that the insurance subsidies — awarded in the form of tax credits — must continue no matter what. That’s what the CRS report concluded. “The funds for such credits would continue to be available via this permanent appropriation during a government shutdown,” CRS wrote.

And because the exchanges are the venue for determining eligibility for subsidies and delivering them, they’d probably have to keep running, as well.

“It may be likely that at least some of the eligibility and processing functions associated with the payment of this credit might continue during a government shutdown,” the report said.

But staffing problems could arise in a long shutdown since agencies are required to furlough all employees deemed nonessential. The diminished workforce could make it harder to make sure the exchanges are operating in the long term.

Furloughs could also affect the state-run exchanges since they’ll still depend on the federal government to help deliver eligibility information. But the effects would probably be less severe because state-run exchanges have to wean themselves from federal operational funds by 2015, when they’re required to become self-sustaining.