President Obama's surprise endorsement of Hillary Clinton on Thursday -- surprise only in the sense that it was sort of sprung on everyone -- made very clear what role Clinton hopes he'll play in the race over the short term. Obama's job is to get Democrats in line behind her candidacy. And, more specifically, it's to get young Democrats on board.

Young Democrats backed Bernie Sanders by even wider margins than they backed Obama eight years ago. Averaging exit poll results for states where they're available, Sanders beat Clinton by 43 points among those under the age of 30.

But young Americans have also been the group that's most supportive of Obama's presidency -- and have become much more supportive over the last year. According to Gallup, 65 percent of those under the age of 30 viewed Obama's job performance favorably in their most recent weekly average. That's 14 points higher than Americans on the whole, and 25 points higher than those 65 and older. Three years ago, the difference between young voters and Americans on the whole was half as large.

So in the video announcing his endorsement, Obama made a very pointed appeal to the overlap of young voters and Sanders supporters.

It's a three step argument.

Step one: Obama knows hard fights, and knows that the party can move past them. In other words: Equate Clinton's victory (and Clinton) with his own (and with himself).

Step two: Acknowledge that young voters played a critical role in the election.

Step three: Point out that Sanders and Clinton share the same values.

It's ... not subtle. But there's probably no better Democrat to make the case to young people for why they should support Clinton than Obama. Even Bill Clinton is only viewed favorably by 48 percent of those under the age of 35, according to recent Quinnipiac polling. Obama's the guy to turn to.

It's also a reminder why this attack might not land that well.

"Obama plus" probably appeals to an awful lot of young Democrats who cast their votes for Bernie Sanders.