Conservatives aren't the only ones who have been having an “I told you so” moment given the rollout problems of President Obama's health care law.

Liberal activists who have long argued for a socialized, single-payer system, are also feeling a certain degree of vindication.

During the health care debate, single-payer advocates felt marginalized. Not only did Democrats rule out a “Medicare for all” type overhaul of the health care system early in the 2009 debate, the major bone they had been thrown (a government-run plan or “public option” to be offered alongside privately administered plans on the new insurance exchanges) was scrapped to get Obamacare passed.

Single-payer advocates had argued that it would have been much more efficient if government simply paid for everybody’s health care instead of mandating people purchase private insurance and then funneling hundreds of billions of dollars in federal subsidies to the insurers. They have been emboldened by the ongoing troubles of Obamacare.

That's why it seems likely that at least one Democrat will run in 2016 and champion the cause of single-payer health care.

One of the most likely politicians to fill that role is Sen. Bernie Sanders, I-Vt. On Dec. 9, Sanders, who has already indicated he was open to running for president, introduced a single-payer health care bill in the Senate.

“As the president fully understands, the rollout has been a disaster, the website has been a disaster,” Sanders told the Daily Beast. “But the truth is, even if all of those problems were corrected tomorrow and if the Affordable Care Act did all that it was supposed to do, it would be only a modest step forward to dealing with the dysfunction of the American health-care system. When you have a lot of complications, it is an opportunity for insurance companies and drug companies and medical equipment suppliers to make billions and billions of profits rather than to see our money go into health care and making people well.”

It’s easy to see this, along with a broader argument about income inequality, being the centerpiece of a Sanders 2016 run.

Though Sanders doesn’t stand a realistic chance of winning the nomination, it will be interesting to see how other Democratic candidates react to such a challenge.

If Obamacare is still a political problem when the primary debates begin in 2015, Democratic presidential candidates will find themselves at a crossroads.

They could either dig in and defend the law as is. They could attempt to blunt attacks from Republicans by introducing some modest changes to Obamacare. Or they could embrace an even more expansive role for government in response to criticism from the left.

Though I don’t think the country will have the appetite for single-payer health care anytime soon, it’s worth paying attention to how this debate plays out.