President Obama is reported to be considering an important brake on the torrent of “dark money” already flooding the 2016 presidential campaign — an executive order requiring federal contractors to disclose their donations to political candidates. Mr. Obama should immediately sign such an order. In doing so he would expose some of the bigger players in today’s big money politics, while offering a healthy counterpoint to Republican efforts to squelch disclosure.

In votes earlier this month, the House Appropriations Committee’s Republican majority quietly inserted an amendment in a spending bill that would block the Securities and Exchange Commission from crafting a rule requiring public companies to open up to their stockholders and voters about their political spending.

Another amendment would stop the Internal Revenue Service from issuing an overdue rule reining in “social welfare” organizations that do not have to disclose donors under current I.R.S. rules and are increasingly misused as big-money conduits for partisan political activity. A third would protect government contractors from disclosing who they’re showering with money.

When it blessed unlimited corporate, union and special interest spending in its fatally misguided Citizens United decision five years ago, the Supreme Court expressed hope that public disclosure would deter corruption. Sunlight, said Justice Anthony Kennedy, would let citizens “see whether elected officials are ‘in the pocket’ of so-called moneyed interests.”