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.

Tax-exempt groups and trade associations spent hundreds of millions of dollars on political advertising during 2012 elections, but they are not required to disclose their donors. Evidence has mounted that a significant portion of the money came from companies seeking to intervene in campaigns without fear of offending their customers, their shareholders — or the lawmakers they target for defeat.

The debate over disclosure poses an early test for Mary Jo White, who was confirmed as the S.E.C.’s chairwoman this month. A new rule would pull the commission into a fierce political battle just as Ms. White and her staff will be wrangling with lawmakers and lobbyists over carrying out legislation imposing new rules on financial institutions.

“We’re keeping an eye on her,” Steve Lonegan, an official with Americans for Prosperity, the conservative political organization founded by Charles and David Koch that has urged the S.E.C. to drop the issue. “My feeling is they are not going to want to deal with this,” Mr. Lonegan added. “The S.E.C. has to deal with its own problems, and with what they’re actually authorized to be doing.”

While campaign finance regulations are usually the province of the Federal Election Commission, advocates for the new proposal have pressured the S.E.C. to issue its own disclosure rule. They argue that shareholders should be able to evaluate business executives’ oversight of company resources and that S.E.C. regulations already require disclosure of similar information, like executive compensation.

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