Pedro Rojas holds a sign directing people to an insurance company where they can sign up for the Affordable Care Act, also known as Obamacare, before the February 15th deadline. (Photo by Joe Raedle/Getty Images)

For the first time, the number of people who have signed up for a health-care plan under the Affordable Care Act has reached 12 million, according to Charles Gaba of ACAsignups.net. The milestone is significant largely because it ends in six zeroes. But it's significant for a political reason, too: It means that 3.7 percent of the country has selected a plan in the politically contentious system.

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First, it's worth parsing the numbers a bit. We spoke with Gaba on the phone so that he could explain the various systems that come into play. The figure includes just more than 9 million people who have enrolled through Healthcare.gov and another just-less-than 3 million signed up under state exchanges. That's official data; Gaba estimates that another 450,000 have signed up but haven't yet been included in official numbers.

Then there was the expansion of Medicaid under the Affordable Care Act. The most recent figures from the government put the total number of people newly covered at 11.7 million. That's only through the end of February, though, so Gaba figures that another 700,000 or so have enrolled since then.

The caveat? Not all of the people who sign up for a plan end up with Obamacare coverage. Last year, 88 percent of people paid at least their first month's premium; if that holds, about 11 million people will have paid for their coverage. But once you factor in policy changes, he assumes that the number of people with actual coverage -- effectuated plans, in the parlance -- will be about 10.1 million through the summer.

The overall figure is relatively constant, but it includes a lot of people dropping and adding policies. "By my estimates, there are still 7,000 people per day enrolling during the offseason," he said, referring to the end of the open enrollment period this spring. That's due to eligible life changes like new children, change of jobs and so on.

OK. So if we combine the 10.1 million people with active coverage under an Obamacare exchange (federal or state) with the 12.4 million enrolled in Medicaid, that's 22.5 million people with health coverage under the Affordable Care Act. That's about 7 percent of the population of the country. A lot may not know that they're enrolled in an Obamacare program, but it still creates a lot of friction for any politician that wants to start pulling aspects of the program away. Which was always the Democrats' political plan, of course.

Gallup has repeatedly tracked the decline of the number of uninsured people in the country, largely through people entering retirement, signing up for Medicaid or buying their own plans. Gaba's data puts a hard number on that last point -- one that just passed another milestone.