(Even if Sanders doesn’t concern you, and you consider Sanders to be the most viable candidate, you should oppose Bloomberg. But that really doesn’t need to be said; most of us on the left already oppose Bloomberg, for ideological reasons. This post is directed to centrists who claim to be opposing Sanders for what they perceive, and with some justification, as the practical reason of his electability.)
Centrists should oppose Bloomberg because Bloomburg is basically a centrist version of what centrists claim to be the most concerning about Sanders.
You dislike Sanders because he’s never run for office as a Democrat. Quite the contrary: he’s run as an independent or a socialist, and never been shy about why he cannot consider himself a Democrat. You worry about that past history alienating centrists and career Democrats.
Well, Bloomburg never ran for office as a Democrat: he ran twice as a Republican and once as an independent. Start worrying about that past history alienating leftists, liberals, and even quite a few plain old career Democrats.
If you worry about Sanders’ lack of appeal because he is too far left, you should worry about Bloomberg’s lack of appeal because he’s too far right.
If you worry about Sanders’ past history of branding himself as “socialist,” you should worry about Bloomberg’s past history of branding himself as “Republican.”
In one candidate — Bloomberg — we have additively combined the significant risk levels of both Sanders and Biden, two candidates that are already troublingly risky.
Bloomberg is thus an exceptionally risky candidate choice. It takes the standard (and very plausible) risk of running a centrist (remember, Democrats, you ran a centrist against Trump in 2016 and she lost), and raises that to the Nth power.
Of all the candidates that have even a ghost of a chance, nominating Bloomberg is probably the surest-fire way to get four more years of Trump.