The biggest opportunity of Mayor Bloomberg’s presidential campaign is the chance he has to make sure that capitalism has a future inside the Democratic Party. Some others are also making tentative, welcome steps in that direction.

In this month’s Democratic presidential debate, Senator Cory Booker urged the party to talk about how to increase prosperity, not just about how to raise taxes. “When I stood in church recently and asked folks in a black church how many people here want to be entrepreneurs, half the church raised their hands,” Mr. Booker said. He advised that the party “start talking not just about how to tax wealth, but how to give people more opportunities to create wealth, to grow businesses.”

This week, another presidential candidate, Mayor Buttigieg of South Bend, unveiled a proposal for what he calls “Public Option 401(k)s.” The plan isn’t perfect, but one intriguing feature is that it would default retirement investment accounts into “life-cycle index funds that combine stocks and bonds.”

Turning more Americans into corporate shareholders, and tying their retirement security to corporate profits, could help reinforce the idea when American companies prosper, that’s a good thing, not a signal that it’s time for government to step in and prevent further money-making. That is an idea desperately in need of reinforcement among Democrats these days.

What Mr. Bloomberg has to offer over and above Messrs. Booker and Buttigieg is that he’s actually created wealth. The first bullet point in the closing frame of his presidential campaign announcement video is “jobs creator.” The video has a narrator explaining that he “built a business from a single room to a global entity creating tens of thousands of good paying jobs along the way.”

This isn’t just standard campaign video gauziness. Mr. Bloomberg understands all this personally in ways that career politicians or law professors just don’t. Mr. Bloomberg started out on Wall Street at Salomon Brothers in the summer of 1966 counting securities by hand in his underwear in an un-air-conditioned vault.

When founding his company, the future mayor spent his weekends dragging cables under customers’ desks in 100-degree heat (no air conditioning on summer weekends) “amid old McDonald’s hamburger wrappers and mouse droppings,” working till 10 p.m. or 11 p.m., according to Mr. Bloomberg’s autobiography.

He once hired Eugene Scalia, now President Trump’s labor secretary, to try to fight off excessive regulation of his company during the Obama administration.

There are plenty of policy points on which reasonable people may disagree with Mr. Bloomberg. Yet his defense of capitalism is fills a needed gap. Here he was speaking in January 2019 in Northern Virginia: “I know it’s very popular among lots of people to say, ‘Oh, I don’t like capitalism, I don’t like business.’ But we all need jobs. We all need an economic base.” Bloomberg said that helping the needy depends on “money that is generated by commerce.”

American capitalism is stronger when two parties, not just one, are in favor of it.

Mr. Bloomberg, I realize, is hardly Milton Friedman. When, early in Mr. Bloomberg’s mayoralty, I asked him during a City Hall interview if he’d consider privatizing New York City’s subways, he replied by asking me what I was smoking. But compared to Bernie Sanders or Elizabeth Warren, he’s practically Ayn Rand.

Any capitalists inclined to snipe at Mr. Bloomberg over his impulses to regulate large sodas, smoking in bars, salt and trans-fat in food, guns, and carbon dioxide emissions would be better off reserving their criticism until after the Democratic primary campaign has concluded. Until then, the Eagle Scout-turned-entrepreneur is the best hope that free-market capitalism has in one of America’s two main political parties.

If Mr. Bloomberg can muster a defense of the system of voluntary exchange of value that made him his fortune — defend it not only on the basis that it creates jobs and generates tax revenue for the government, but on the moral ground that it rewards hard work and risk-taking and incentivizes innovation and serves customers better than any government program — he and our country will be winners no matter what happens in the election.