On homeland security, people say that we have to be on top of things 100 percent of the time, while our foes need only be successful once. The need for perfection is daunting, especially when operating with a large bureaucracy in which things fall through the cracks.

 REUTERS/Kevin Lamarque President Obama (Kevin Lamarque/Reuters)

Imagine then how the White House must approach Obamacare. It can madly try to lower expectations — only 80 percent of the people will get through on HealthCare.gov by Dec. 1 — but in fact it’s run out of slack and one more noteworthy event is likely to send the Democrats running for the hills. In other words, the gang that got practically nothing right now has to get virtually everything right.

Megan McArdle writes:

The moment that it looks like there’s a big risk that Obamacare won’t work, both Democrats and insurers are going to stampede for the exit. Yes, Obama can veto anything that threatens his favorite law. But if it gets that far, he’s already lost. . . .

The law would still be nominally in effect until 2017, but coverage would drop and get more and more expensive. By the time a Republican president came to town, it would be clear that most of the Patient Protection and Affordable Care Act was slated to be repealed in a bipartisan reform.

Democrats and insurers are at a very delicate moment right now. If they can hold it all together until after the 2014 election, then they may well be able to cement this law into place. But November 2014 is still a long way away. And every time a mainstream news outlet suggests that the law might not survive, the odds of that happening increase.

What could send Obamacare over the cliff? More horror stories after the Web site is “fixed.” Few if any new sign-ups. A security lapse. Or maybe this:

Henry Chao, the Obama administration official who oversaw the technical development of the federal health insurance marketplace, said Tuesday that his team has yet to complete 30 to 40 percent of the overall project.

Speaking before a subcommittee of the House Energy and Oversight Committee, Chao said the Centers for Medicare and Medicaid Services is still working on a number of “back office” aspects of the project, including a system to send payments to insurance companies.

Soak that in. There is currently no mechanism to transmit payment to insurance carriers. (Are they supposed to operate on a handshake if they haven’t gotten the money from the feds?). That isn’t all: “Also yet to be finished is a component that ensures that the state and federal marketplaces and the insurers have accurate, matching information about enrollments. Officials are also still working on a system that makes payments to insurers that attract high-risk patients. These systems must be in place by January, officials have said.”

Let’s review then what cannot happen in the next couple of months:

No security breaches.

No more stories of interminable waits on HealthCare.gov.

No incomplete accounting systems.

No significant sticker shock. No increase in adverse selection.

No hitches in re-upping insurance for those plans cancelled previously due to Obamacare.

And no complaints about people losing their doctor because he or she is not on the subsidized plan available on the exchanges.

The last may be the most significant and the final straw for many people. It is one thing to lose the plan you liked because it was cost-effective; it’s another to be told your doctor won’t be covered by the new plan you didn’t want to buy in the first place.

It is not a matter of GOP talking points or votes. It no longer matters what the White House or the critics say. It comes down to reality. Either all these pitfalls are going to be essentially eliminated or they aren’t. If not, there is virtually nothing that will save Obamacare from implosion.