It began as a lark. Now it's an official rumor: Hillary Clinton might run for mayor of New York against Bill de Blasio in 2017.

It would, some argue, be a fitting coda for a life of government service. And, some of the more politically minded supporters of this idea note, it would be a fitting revenge on de Blasio, who, despite running Clinton's 2000 Senate campaign, was very wishy-washy in the early days of the 2016 presidential race as he sought to position himself as a national voice for liberals.

What's strange about all of this is not that the speculation exists. It's that Clinton isn't publicly ruling it out. This, from Politico on Tuesday:

Why would she knock it down, people who know her ask. She still wants to be relevant, and there’s nothing wrong with speculation. The idea that people want to draft her is flattering, particularly after an embarrassing loss, they say. Plus the reality: If she ran, she’d probably be so immediately dominant that de Blasio might have to just step aside rather than get crushed on the way to her victory.

I get, from a personal perspective, why the mayoral speculation appeals to Clinton. She is coming off not only a loss but an entirely unforeseen loss — her second in as many tries for the White House. She is also someone who has spent a lifetime in public service and, given that she was planning to be president, will look for other ways to continue her service.

But the idea that Clinton would walk into the office, perhaps not even having to run against a cowed de Blasio, who would step aside rather than face a massive loss at her hands, seems less than certain. It could happen. It might be the most likely outcome. But, as Dave Weigel so ably noted in this space recently, there is a tendency where Clinton is involved to glorify the idea of her running and then turn on her once she actually becomes a candidate. The simple fact is that if Clinton ran, there would be a series of "she's just obsessed with running for things" stories. And those would be followed by a series of "does she even care enough about New York to run it" stories. You get the idea.

Although that tendency is more pronounced with Clinton, it is not unique to her. Mitt Romney faced a similar "you should run/how could you run" dynamic in his political career and, smartly, decided not to run for president a third time, in 2016.

That's one of several ways in which Romney is a good case study for Clinton as to why she absolutely should come out and say publicly that she is not interested in running for mayor. The other big way is that by stepping out of the process, Romney was empowered to play a sort of senior statesman role in the 2016 campaign. Yes, he used that perch to unsuccessfully try to move his party off of Donald Trump as its nominee. But the reason Romney was taken seriously — and received so much press attention for his efforts — was because he not only had the gravitas of having been a past nominee of the Republican Party but also was someone who had put ambition aside for the good of the party.

That seems to me to be the role to which Clinton is, at this point in her career, best suited. And the role the Democratic Party so badly needs her to play.

President Obama is likely to follow the lead of many past presidents and divorce himself — at least in the near term — from the day-to-day political world after his term ends in about a week. That means that a vacuum of leadership will occur, a vacuum that Clinton is the natural pick to fill. She, unlike Obama, is someone who has spent much of her adult life working in and for various party structures. She is someone who can command an audience with every corner of the party. And she is the sort of person who can help facilitate the hard transition within the Democratic Party from Obama-ism to whatever comes next. (Obviously, Clinton had hoped that what came next would be a return to Clinton-ism.)

Running for mayor — or even continuing to allow speculation about it to fester — makes Clinton, with every passing day, a less valuable Democratic broker. And that is a far more important role than serving as the top elected official in New York City. (Sorry, NYC residents!) Keeping her name in the mix for mayor might provide Clinton some level of personal satisfaction. But it does very little to advance the cause to which she has devoted the majority of her adult life: building (and re-building) the Democratic Party.

That's why Clinton needs to make clear that she won't be running against de Blasio — or anyone else — in 2017.