The Affordable Care Act, a k a Obamacare, goes fully into effect at the beginning of next year, and predictions of disaster are being heard far and wide. There will be an administrative “train wreck,” we’re told; consumers will face a terrible shock. Republicans, one hears, are already counting on the law’s troubles to give them a big electoral advantage.

No doubt there will be problems, as there are with any large new government initiative, and in this case, we have the added complication that many Republican governors and legislators are doing all they can to sabotage reform. Yet important new evidence — especially from California, the law’s most important test case — suggests that the real Obamacare shock will be one of unexpected success.

Before I can explain what the news means, I need to make a crucial point: Obamacare is a deeply conservative reform, not in a political sense (although it was originally a Republican proposal) but in terms of leaving most people’s health care unaffected. Americans who receive health insurance from their employers, Medicare or Medicaid — which is to say, the vast majority of those who have any kind of health insurance at all — will see almost no changes when the law goes into effect.

There are, however, millions of Americans who don’t receive insurance either from their employers or from government programs. They can get insurance only by buying it on their own, and many of them are effectively shut out of that market. In some states, like California, insurers reject applicants with past medical problems. In others, like New York, insurers can’t reject applicants, and must offer similar coverage regardless of personal medical history (“community rating”); unfortunately, this leads to a situation in which premiums are very high because only those with current health problems sign up, while healthy people take the risk of going uninsured.