In the land of political toadstools, former New York City Mayor Mike Bloomberg is a giant. Even without a box to stand on behind the debate podium.

But, boy, was that a shootout Wednesday night in Las Vegas.

Welcome, Mr. Bloomberg, to the 2020 presidential campaign that you are supposed to salvage for Democrats.

It was almost painful to watch all the blows landing on the bewildered Mr. Bloomberg, the savior with the invisible campaign. Last night, Mr. Bloomberg was the Incredible Shrinking Candidate.

Which is a really terrible thing when you start out so tiny.

The whole debate offers an astonishing indictment of the Democratic Party that a guy like Mr. Bloomberg can rocket to the top of the heap without a single day of actual campaigning and in spite of his political record.

Just a few elections back, he endorsed President George W. Bush, a Republican, for reelection. Heck, Mr. Bloomberg himself was a Republican more recently than that.

And in 2012, Mr. Bloomberg begrudgingly endorsed President Obama, a Democrat, for reelection just days before the election. Today, of course, Mr. Bloomberg is running tens of millions of dollars worth of ads claiming he was a crucial right-hand man for Mr. Obama during his presidency.