Though every Democratic candidate is against President Trump and in favor of working Americans, antitrust is an issue over which the candidates have real disagreement. There are stark differences between, say, Senator Bernie Sanders’s calls to “break them up” (usually a reference to banks), and former Vice President Joe Biden’s “cooperative” approach. Mr. Biden (still undeclared), has taken the position that big corporations should not be “singled out” and that their chief executives can be persuaded to shoulder their responsibilities toward workers and communities. (In the late 1970s, Mr. Biden resisted efforts to strengthen the antitrust laws, though his views may have changed as the law has grown weaker.)

Antitrust law is not an instrument of socialism or of unfettered capitalism; it seeks to protect markets from abuse by their participants. Ms. Warren and Senator Amy Klobuchar seem to have the deepest understanding of this. Ms. Klobuchar is not as aggressive on the issue as Ms. Warren, but she has cast herself as an antitrust reformer, introducing thoughtful new laws for merger reform. Senator Cory Booker, for his part, has over the last year staked out a trustbusting position centered on labor markets, with particular emphasis on the worrisome effects of corporate consolidation on workers and their salaries.

The largest question mark among the major candidates is Senator Kamala Harris of California, who represents the state where the largest tech monopolies are headquartered. She was tough in her questioning of Facebook’s chief executive, Mark Zuckerberg, during congressional hearings last year, but she has been silent on the questions raised by tech monopolies.

Ms. Harris attracted some attention last month when she declined to register opposition to the proposed merger of the wireless carriers Sprint and T-Mobile — in stark contrast to the rest of the senators running for president. It may be that she is relatively open to approving mergers; it is also possible that her positions are just undeveloped.

The variety of antitrust positions in the emerging Democratic field means that it will no longer be enough for a candidate to mutter a few platitudes about big corporations and let the party’s technocrats decide what the nation’s approach to monopoly power ought to be. Such questions of economic policy affect us all and therefore should sit at the core of a majoritarian democratic process.