A loose coalition of Democratic elected officials, shareholder activists and pension funds has flooded the Securities and Exchange Commission with calls to require publicly traded corporations to disclose to shareholders all of their political donations, a move that could transform the growing world of secret campaign spending.

S.E.C. officials have indicated that they could propose a new disclosure rule by the end of April, setting up a major battle with business groups that oppose the proposal and are preparing for a fierce counterattack if the agency’s staff moves ahead. Two S.E.C. commissioners have taken the unusual step of weighing in already, with Daniel Gallagher, a Republican, saying in a speech that the commission had been “led astray” by “politically charged issues.”

A petition to the S.E.C. asking it to issue the rule has already garnered close to half a million comments, far more than any petition or rule in the agency’s history, with the vast majority in favor of it. While relatively few petitions result in action by the S.E.C., the commission staff filed a notice late last year indicating that it was considering recommending a rule.

In response to the growing pressure, House Republicans introduced legislation last Thursday that would make it illegal for the commission to issue any political disclosure regulations applying to companies under its jurisdiction. Earlier this month, the leaders of three of Washington’s most powerful trade associations — the U.S. Chamber of Commerce, the National Association of Manufacturers and the Business Roundtable — issued a rare joint letter to the chief executives of Fortune 200 companies, encouraging them to stand against proxy resolutions and other proposals from shareholder activists demanding more disclosure of political spending.