One of the big mysteries about the government shutdown is why Republicans are so furiously opposed to the Affordable Care Act, a.k.a., Obamacare.

Yes, the Act has many problems, not the least of which are its high costs and mind-boggling complexity.

But the Act's overall goal, which seems achievable, is to make health insurance more affordable for tens of millions of Americans. And although one can be frustrated about some details, it seems strange that this should cause a party to rage and howl and shut down the government. (On the contrary, it seems like the sort of thing that a certain kind of leader might justifiably be proud of — helping his or her constituents and their families gain access to something so basic and important.)

So why are the Republicans so outraged about Obamacare?

There are different theories.

The Republicans themselves say that Obamacare will destroy the country. They rarely explain that assertion, though, so it's hard to know what they actually mean.

They say that Obamacare will destroy the economy, by forcing businesses to spend more on health care costs and compliance and, in some cases, by hiring part-time workers instead of full-time ones. This may be true at the margin, but companies have long been spending more on health care and reducing health care benefits for their employees while shifting more of the costs onto them. (This is in part because our current system, in which employers pay for health care, is deeply flawed.) But the truth is that, for most companies, Obamacare just isn't that big a deal. I run a company, and that's the way I would characterize it: It's a little deal but not a big deal. It's not going to cause us to change our hiring plans or fire anyone.

They say that Obamacare violates the Constitution. Lots of people said this after the government voted Obamacare into law, but the Supreme Court said that they're wrong. So this argument doesn't hold much water, either.

Outside the Republican Party, there are different theories.

Some say the Republicans just hate anything that Obama likes. Based on the last five years, there's seems to be plenty of evidence to support this theory. But there seems to be even more than that going on here.

Another theory, popular last year, is that the Republicans don't like the tax increases that are necessary to pay for Obamacare. But the Republicans don't talk about these taxes (or the deficit) anymore when they air their objections to Obamacare.

Another theory is that the Republicans are just "crazy." Although it does seem odd to be so rabidly opposed to something that will likely help so many Americans, writing a whole political party off as "crazy" seems like a throwaway.

So why do the Republicans hate Obamacare?

Eduardo Porter floats an interesting theory in the New York Times.

The Republicans hate Obamacare, Porter suggests, because they're terrified that Americans are going to like it.

Specifically, they're terrified that some of the tens of millions of Americans who will have greater access to health insurance under Obamacare will realize that the new law is actually not going to destroy the country — that, in fact, it is going to improve life for tens of millions of Americans, including, importantly, many who vote Republican.

Many of the Americans who will benefit from Obamacare, Porter points out, are relatively poor red-state Republicans who are currently devout supporters of the Republican Party. For decades, the Republican pitch to these voters has been that "less government is better." If Obamacare works the way it is supposed to, however, it's possible that "more government" might actually seem to be better — and that many die-hard Republican voters might realize that.

After all, even now, before Obamacare has really gotten going, Americans actually very much like the specific benefits of Obamacare — as long as you don't call it "Obamacare."

As Michael Hiltzik reported in the LA Times yesterday, when Americans are asked whether they like "Obamacare," a majority say no. But when Americans are asked whether they support what Obamacare actually does, they love it:

Here are figures from Kaiser's March 2013 poll:

Tax credits for small businesses to buy insurance: 88% in favor.

Closing the Medicare drug benefit doughnut hole: 81% in favor.

Extension of dependent coverage to offspring up to age 26: 76% in favor.

Expanding Medicaid: 71% in favor.

Ban on exclusions for preexisting conditions: 66% in favor.

Employer mandate: 57% in favor.

What Americans don't like is the "individual mandate," which forces them to buy health insurance. This is understandable — no one likes to be forced to do anything. And it's true that this provision is probably what gives the Republicans their most persuasive firepower in attacking the law. But the big reason the "individual mandate" is in there is that all the other benefits would be entirely unaffordable without it.

If more Americans end up liking Obamacare, including some Americans who have bought into the "less government is better" pitch, this would be bad news for the already increasingly marginalized Republican Party.

So that's certainly one interesting theory about why the Republicans hate Obamacare so much — because they're worried that their constituents are actually going to like it.

But throwing away old positions and embracing new ones is a big part of politics. So maybe the Republicans should stop fighting "Obamacare" and just call it "Cruz-care."