Former New York City Mayor Michael Bloomberg has thrown his hat in the ring for the 2020 Democratic nomination, and the conventional wisdom is all ready with a fusillade of mockery. Were Democrats clamoring for another white man over 75 to join the race? Have moderates worried about the party's left-wing drift finally found a candidate who Sen. Bernie Sanders (I-Vt.) supporters can comfortably reconcile themselves to come November? Is mayor "stop and frisk" the man to lure African-American voters away from the sweet but flailing former Vice President Joe Biden? As for the general election, clearly the only way to beat a fake billionaire is with a real billionaire, right?

But rather than being greeted with derision, his candidacy should be met with an argument — and the Democratic Party should be thankful for the opportunity to have that argument before the primary is done.

Bloomberg is typically — and accurately — described as a centrist. But that description is inadequate. Like a triangle, politics has multiple centers. And they can be far enough apart as to be pushing in opposite directions.

The center is most simply described as a point between the two parties, rejecting the extremes and embracing compromise. That's not an unpopular place for a politician to be, but the left-wing critique of this kind of centrism (like the very similar right-wing critique) has real teeth: By defining itself against the extremes, it allows itself to be defined by the extremes. A party that embraces this kind of centrism as its identity allows the other party, if it chooses to stand on principle, to drag the center further and further in its direction. That's the problem with the idea of anointing someone like Sen. Amy Klobuchar (D-Minn.) as the avatar of the moderate wing: Her centrism lacks ideological content, and therefore lacks driving force.

No one can say that about Bloomberg. Indeed, he is one of the most accomplished representatives of what we might call elite centrism. This perspective favors the neoliberal consensus in economic policy: trade liberalization, low inflation, and deficit reduction achieved through a mix of tax increases and entitlement cuts. It favors continuity in foreign policy with the post-Cold War consensus: expanding and strengthening NATO, wiping out terrorist groups, and both containing and engaging China. And it favors a liberal but not hyper-liberal approach to social policy: pro-choice, pro-gay-rights, pro-immigration. A simple way of describing this perspective would be: How can we do more of what America has done well lately, and get it to work well for more Americans?

This is not an ideologically-neutral centrism that splits the difference between left and right. It's a coherent perspective of its own. And it can enact sweeping change, for better or for worse: Just look at what Bloomberg did to New York's school system, where he dramatically expanded high-stakes testing and programs of public school choice. His campaign will likely show similar vigor, emphasizing not only his steady, experienced hand but advocating strong government action on certain issues, like climate change, even as he takes a more moderate tack on others, like health care. Meanwhile, the fact that Bloomberg was able to win three terms as mayor in an overwhelmingly Democratic city is a testament to the fact that, in the right circumstances, this elite centrist perspective can be quite popular.

But Bloomberg's is not the only center. There's another center — the populist center — that diverges from both left and right in entirely different ways from elite centrists. It favors a more generous social safety net (albeit one designed to reward work), higher taxes on the rich, and an industrial and trade policy aimed at rebuilding America's manufacturing strength. It favors a foreign policy that is both more nationalist than either the left or elite centrists are comfortable with, and less aggressive than either the right or elite centrists advocate. And it is instinctively more conservative on social issues, particularly questions of identity. This is the center that powered Ross Perot's independent candidacy in 1992, that Trump sometimes claimed to occupy, and that wooed enough moderate Republicans to win Trump the primary, and enough disaffected voters and "Obama-Trump" voters to prevail in the general election.

The question for 2020 Democrats has often been described as whether they should move to the center or move left — whether they should focus on winning swing voters or energizing those who stayed home in 2016. But this way of phrasing the question presumes that "moving to the center" has a singular meaning, when in fact, the question of which center to move towards is just as important. Should Democrats argue that Trump is an incompetent charlatan who did not deliver for American workers or reduce America's military commitments? Or should they argue that he is a radical who threatens the stability of America's political system, economy, and place in the world? Should they try to unite the left with moderate Republicans who have been horrified by Trump in practice? Or should they try to win back "Obama-Trump" voters and voters who had been dormant but came out for Trump?

That's where Bloomberg can make a real contribution. The value of Bloomberg in this debate between rival centers is that he comes down resoundingly on one side. Bloomberg's record, identity, and predisposition all make him an exceptionally strong proponent of the elite centrist view. As such, his presence in the debate could force the other candidates — particularly the others also vying for the center like Biden and South Bend, Indiana, Mayor Pete Buttigieg — to clarify their differences with him, and make a case for a more capacious center that is not merely a reaction to Trump but can address some of the concerns that powered his campaign.

If he can do that, he'll have improved the prospects of whoever wins the Democratic nomination. Because that person will have to unite not only the left with the center, but these different and opposed perspectives that each see themselves as defining the natural center of our national life.

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