Suppose, for a moment, that you worked as a staff member to a Democratic member of Congress, or perhaps in the Obama administration, or in the world of academics and think tank experts advising both.

Perhaps you worked countless all-nighters on the language of the Affordable Care Act or the Dodd-Frank Act — or maybe you were at an agency trying to write the thousands of pages of regulations to institute those laws, or even an advocacy group trying to nudge all of the above to the left.

You know the compromises that were made back in 2010 and why — uniting 55 or 60 senators with wildly different political temperaments and local politics was really hard. You had to come up with a bill that could get a “Yes” vote from both a centrist like Joe Lieberman or Joe Manchin and, well, a democratic socialist like Bernie Sanders.

You’re convinced that those laws — much hated by both conservatives and the industries they overhauled — made the United States a better place, helping millions more people afford health care and reining in the financial industry. You know the laws aren’t perfect — but also believe that future presidents and Congresses should build on them, much as Social Security and Medicare are now much expanded from their original charters.

Now comes a man who has had to answer only to voters in the most liberal state in the nation, who has never had the responsibility to actually pull together the disparate center-left coalition that is the Democratic Party to enact concrete legislation.

When Mr. Sanders argues for scrapping Obamacare’s intricately constructed mix of private health insurance with public subsidies for a single-payer government program, he’s essentially saying your efforts were useless, hopelessly corrupted by the health insurance industry. Same with Mr. Sanders’s call to break up the largest banks, as opposed to the current approach of just regulating them more intensively.