In it, but not of it. TPM DC

CHART: Obamacare's March Enrollment Explosion To Hit 8 Million

AP Photo / Charles Dharapak

Enrollment sat at 4.2 million at the end of February, but exploded to 8 million by April 19, the new report said. Among those ages 18 to 34, the crucial young and invincible demographic, sign-ups doubled from 1.1 million to 2.2 million.

With enrollment closed, that group accounted for 28 percent of enrollees: lower than the administration was hoping for, but at a high enough level to sustain the law, according to health policy experts.

The 36 states using had a particularly good month: Sign-ups more than doubled in the final stretch, from 2.6 million in the first five months to 5.4 million as of April 19.

President Obama had already announced the 8 million figure, but Thursday's report included more specific data, included a state-by-state breakdown.

HHS March Enrollment Report