The news that Michael Bloomberg might compete for the Democratic presidential nomination is causing consternation on the political left. But that’s all the more reason to welcome his candidacy to challenge a vulnerable President Trump next year.

“Memo to Bloomberg: Democratic Voters Don’t Want More Candidates” blared a headline Thursday night on the left-wing Huffington Post urging the former New York Mayor to stay out. The piece was the first we saw of what will be many lecturing Democratic voters that they should be happy...