The launch Sunday of Mayor Bloomberg’s national advertising campaign favoring the Democrats is being taken as a step toward his running in for president in 2020, this time as a Democrat. We hope he goes for it. At a time when the Democratic Party is careering leftward toward outright socialism, we agree with the New York Post that His Honor has a chance to “bring Democrats back to sanity.”

This isn’t the first time … strike that, this isn’t the second time … strike that, this isn’t even the third time we’ve encouraged the world’s 11th richest person to throw his $50 billion hat into the ring. In 2008, we savored the possibility of a three-way race, between Mayor Giuliani, Senator Hillary Clinton, and Mr. Bloomberg. It might have boosted The New York Sun’s print circulation.

We called such a race the “New York Central.” Elliott Banfield drew a wonderful cartoon. At the time Mr. Bloomberg, who’d won the New York mayoralty as a Republican, was thinking of running for the White House as an independent. Then he had a rendezvous with Hamlet, who talked him out of making up his mind. The race fell to Senators McCain and Obama, and the rest is history.

Mr. Bloomberg’s explanation at the time was that he didn’t think he could win. His chances today may be even less. The Post’s point — and we agree — is that one never knows, particularly after President Trump rode his escalator into history. We’ve long since said our l’envoi to the Democratic Party, but we confess that we miss the party as it once existed.

That was the party that Ira Stoll wrote about in “JFK, Conservative.” He was Democrat who was pro-life, pro-civil rights, and anti-Communist, even if he made such blunders as countenancing the assassination of Vietnam’s anti-communist premier, Ngo Dinh Diem. JFK’s big economic move was to cut tax rates at the top margin, setting the stage for Reagan and the boom that followed.

It’s hard to imagine Mr. Bloomberg getting anywhere with a campaign against his hobby-horses of guns, soft drinks, and cigarettes. He opposed for the Supreme Court even such a moderate as the Chief Justice, John Roberts. It’s at least possible, though, to imagine Mr. Bloomberg illuminating the illogic of the Democrats’ penchant for tax increases. His business empire, like Mr. Trump’s, is global.

One virtue of the independent campaigns that tempted Mr. Bloomberg in 2008 and a bit in 20012 was that they could have leached votes from the Democrats. In 2020, if Mr. Bloomberg lost the Democratic nod after a respectable showing against a left-winger, he could still mount an independent run. Imagine Mr. Trump for the GOP, Andrew Cuomo for the Democrats, and Mr. Bloomberg as an independent. The New York Central rolls again. Aaaaall aboard.