Bloomberg can easily handle Trump, and with Klobuchar or Mayor Pete or Warren or whomever, the Dems will accept an "anybody but Trump."
Bloomberg can handle Trump in debates (supposing that Trump will accept debates; likely he won't, and shouldn't, if he is smart), but Bloomberg can NOT attract Bernie's voters, and Bernie's voters are at least 25% of all Democrat voters.
In the last several presidential elections the difference between the Dem and GOP candidates was small. You simply CANNOT win the general election if you alienate 25% of the people who might supposedly vote for you.
Bloomberg if he wins the nomination will seduce all moderates on the Dem side. He won't seduce the moderates on Trump's side because those will vote for Trump no matter what. Trump's side is united; it is a cohesive bloc. So, Bloomberg needs the Dem moderates PLUS the progressives, to edge above Trump. But he won't have ANY progressives, so, he is toast.
I think Bloomberg is the least electable of all candidates because there is no fix for him. Short of getting as veep Michelle Obama (which will NEVER happen) or Sanders himself (who will NEVER join forces with a Wall Street-friendly billionaire) he can't fix the problem, so, he WILL lose ALL progressives in November. He simply can't win in November without the progressives.
All others might even be somewhat viable if they pick veeps who supplement them and compensate for their weaknesses. But Bloomberg can't do it. The perception that he is buying his way into the nomination is so turn-offish for progressives that even running against Trump, I'd be surprised if any true progressive voted for him.
And then, his path to the nomination is very unlikely to go through a simple, clean, and fair majority set of pledged delegates who will deliver him the victory in first convention ballot, simply because even if he grows and grows, he won't be able to attract ANY of the support for Sanders who travels on a different lane. Therefore, given that the Dems allocate delegates proportionally, it is highly unlikely that Bloomberg, starting late, will earn more than 50% of pledged delegates in the first ballot. Sander's delegates and some other delegates for other candidates will keep Bloomberg below 50%.
Therefore his ONLY path to the nomination, is to count on the Party's establishment and superdelegates, to give him the victory in second round (remember, this time, superdelegates only vote starting with the second convention ballot).
Now, if this happens, Sanders' supporters will be pissed off, 100% outraged, and a lot of people will feel cheated as well even if they aren't Sanders' supporters.
An illegitimate nomination for Bloomberg will be perceived as a back room deal filled with fat cats smoking cigars. Conspiracy theories will pop up saying that he used his billions to bribe people into submission. His support will plummet. He may end up with only half of the Dem voters, not even 75% (after irreversibly losing the 25% Bernie people, he'd lose even more people by being perceived as a cheater).
Good luck winning the general election against Trump, with only 50% of Dem voters supporting him.
So, if the Trump campaign is trying to run against Bernie, if they are smart they should switch and try to run against Bloomberg. Billionaire vs. Billionaire? Most leftist Americans younger than 30 will sit out. Essential blue colar workers in the Rust Belt will sit out.
In the unlikely event that Bloomberg wins the nomination fair and square with a majority of pledged delegates in first ballot, obtained through winning enough primaries and caucuses, he'd be my least preferred Dem candidate but I'd vote for him against Trump, sure.
If he buys his way into the nomination and is handed the nomination on a silver platter through an unfair brokered convention, even I, one of the most anti-Trump people out there, won't vote for Bloomberg. Of course I won't vote for Trump either, but I will sit out or vote third party.
Sure, I've said that doing that is indirectly voting for Trump, but in the case of Bloomberg, no, it isn't, because he won't win anyway without Bernie's 25%, so the election would be all but decided in favor of Trump; may as well teach the Democratic Party a lesson by going third party.