That means, first, that proposals should meet a baseline degree of factual plausibility — a bar that, for example, the Medicare-for-all plan that Mr. Sanders and Ms. Warren favor does not clear. Ms. Warren’s Tuesday night zinger was aimed at former congressman John Delaney (Md.), who had pointed out correctly that the numbers behind the proposal simply do not compute: The senators cannot deliver a system that provides far more benefits than other single-payer systems they claim as their model while preserving the level of care and access that insured Americans currently enjoy. They should make the case for a government monopoly on health care if they want, but they should be honest about the trade-offs.

Candidates who promise big ideas should also be pressed on how they will realize them. Mr. Sanders says he will lead a revolution. Ms. Warren will take on the “giant corporations that have taken our government and that are holding it by the throat.” Then, the theory goes, they can bring about radical change.

But the United States is a vast, pluralistic country, and Congress will continue to reflect its ideological range. Big donors and billionaires may exercise too much influence, but Democratic primary voters should be wary of candidates who use that fact to explain away all opposition to their ideas. Even if you undid Citizens United and enacted campaign finance reform, sustainable policy in America would emerge only by means of principled compromise.