Maybe Bloomberg will join the race later on; that might be interesting.
Bloomberg is already running. It's an unconventional campaign, based primarily on pouring enormous amounts of money into mass media―conventional and Internet (and one thing Bloomberg certainly knows is media)―rather than on retail politics, although he is putting in a fair amount of time meeting voters face-to-face and he has assembled a large and, reportedly, unusually well-paid campaign staff. It should be clear after March 3, when 14 states* conduct intraparty "primary elections," whether he has a reasonable shot at becoming the Democratic Party candidate.
Bloomberg is hardly charismatic, but he is smart, a policy centrist, his campaign is data-driven and therefore flexible, and he has the financial and staff resources to produce the high voter turnout the Democrats need to overcome the Republican advantage in the electoral college. Plus, while he may lack charisma, I suspect he could run rings around Trump verbally in a debate setting―and that he will know how to utter the magic incantations that will drive el seƱor Loco even more loco. (When asked by a reporter the other day whether it was a good idea to have two billionaires running against each other for president, Bloomberg dryly replied, "Who's the other billionaire?")
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*Alabama, Arkansas, California, Colorado, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah, Vermont, Virginia, plus a special primary for U.S. expats.