Robert B. Doherty, senior vice president of governmental affairs at the American College of Physicians, wrote this for "The Field Clinic" on Philly.com

Much of the news lately about the Affordable Care Act - Obamacare - has been about the troubled rollout of the Web marketplace enrollment and the cancellation notices going out to a segment of the 5 percent of Americans with plans in the individual insurance market. But another Obamacare story has gotten far less attention: enrollment in Medicaid, which is being expanded in many states, is going gangbusters.

Nine out of 10 new Obamacare enrollees have signed up for Medicaid, the Washington Post reports, compared to only "a trickle of sign-ups for private insurance."

This is potentially a watershed development, because Medicaid is Obamacare's only true public option: a program jointly run and funded by federal and state governments. But Medicaid, which even before the ACA was the nation's largest insurance program with 62 million enrollees, is central to Obamacare's goal of insuring nearly all Americans.

Under the ACA as originally written by Congress, Medicaid eligibility would have been expanded to all persons with incomes up to 138 percent of the federal poverty level (about $27,000 for a family of four), paid almost entirely by the federal government. States that didn't go along would have been penalized by losing all their current Medicaid funding.

But the Supreme Court, in its decision last year to uphold the rest of the ACA, said that it was unconstitutional to impose penalties on states that don't expand Medicaid, making it voluntary; states can take or leave it.

Perhaps it is not so surprising if Medicaid enrollment soars in the 21 states, including New Jersey, that have agreed to expand eligibility. But Medicaid enrollment is likely to rise even in states such as Pennsylvania that have not yet agreed to the expansion, because the ACA makes the Medicaid enrollment process easier in all states. The non-expanding states will get more Medicaid enrollees but without any extra federal expansion money. Pennsylvania Gov. Tom Corbett has proposed using the federal money to enroll people in private insurance plans offered through the Obamacare marketplace, although this would require approval by the legislature and then the federal government. Some experts doubt that his "private option" will save the state money.

The lopsided enrollment trends in Medicaid compared with private insurance may change when the www.healthcare.gov enrollment site is fixed.

It is also possible, though, that we are seeing what liberals have long sought and conservatives have long feared: that if people are offered a chance to easily enroll in a public plan (Medicaid) that generally offers better benefits and costs less than private insurance, they will jump at it, as opposed to navigating a maze of private plans that offer less and cost more.

If this is so, conservatives might rue the day that they rooted for Obamacare's website to fail. Rather than proving that the government shouldn't be more involved in health care, a failure of the ACA's private insurance enrollment process might make the case for further expanding Obamacare's other, simpler public option, which is to make it as easy as possible for people to sign up for Medicaid. In other words, single-payer health care.