Those 4.6 million people have skewed young and non-white. The drop since the fourth quarter of 2013 among those aged 25 to 34 was 7.4 percentage points. Among blacks, 7.3 percentage points; Hispanics, 8.3 percent percentage points. Among those earning less than $36,000 a year, it's 8.7 percentage points.
In other words: It's the Democratic base.
Earlier this year, we looked at the longstanding charge that Democrats "buy" votes with social welfare programs, and found that it's simply not the case. But Obamacare was long been predicted to pay long-term dividends for the party. As recently as December 2013 -- after the botched roll-out of the exchange Web site -- then-Senate Majority Leader Harry Reid (D-Nev.) predicted that the program would help Democrats hold the Senate. As his current title of Senate minority leader makes clear, he was incorrect.
In 2016, though? Could be, for two reasons.
First, polling shows that opposition to the bill fluctuates within a narrow range. It spiked last summer, but in March of this year was only two points higher than approval. It's important to the Republican base that will vote in primaries, but it doesn't seem to be getting much more unpopular.
The second reason is more important. Historically, younger voters and non-white voters vote less heavily than older, white voters. What's more, lower-income voters also vote less frequently than wealthier ones. It's a big problem for Democrats, but one that is significantly less of a problem in presidential elections.
The key for any Democrat, particularly one in what will likely be a tough general election fight, is turnout. (It's a cliché, but it's true!) Whoever ends up being the Republican candidate will likely have pledged at some point to repeal Obamacare -- allowing whoever ends up being the Democratic nominee (whoever that might be!) to say, "Vote for me or else." The Republican Senate and House will make that message even more potent.
And some large subset of those 4.6 million people -- a number that Gallup predicts will increase -- will repeatedly see this message: Go vote for the Democrat or risk losing their health-care coverage.
That's a particularly powerful impetus to get to the polls. Which is why we feel comfortable predicting that it will be central to the Democratic nominee's campaign in 2016.