This year the Republican and Democratic nominating conventions in Cleveland and Philadelphia will be bankrolled entirely with money from corporations and wealthy individuals. Not since the Watergate era, when a $400,000 pledge to the 1972 Republican convention from ITT Corporation was linked to a favorable outcome for the company in a federal antitrust decision, has this happened.

Industries with business before the federal government have long found opening their checkbooks for the conventions to be one of the most efficient means for influencing an incoming administration and Congress in one quick action.

Come July, some of America’s best-known companies will pay to celebrate the nomination of Donald Trump, whose racist rants have in the past caused NBC, Macy’s and Nascar to distance themselves from him. Progressive political groups like ColorOfChange.org and Credo Action are pressuring Coca-Cola, Walmart, Microsoft, Facebook and Google to cut off money for the Republican gathering. But the protest against Mr. Trump doesn’t address the deeper problem of corporate influence over both parties.

The ITT scandal prompted legislation that provided public financing for conventions, and limited their budgets to that amount. But the parties soon found multiple ways around that, including using “host committees” that operate in the cities where the conventions are held, soliciting unlimited amounts of convention money from corporations and wealthy individuals. These committees, established to skirt federal laws banning corporations from giving to political parties directly, should be abolished.