Major U.S. companies as diverse as the drugmaker Merck, the chemical giant Dow and the financial services firm Prudential wrote big checks — some for more than $1 million — to the U.S. Chamber of Commerce last year for dues and political activity, according to recent disclosures.

But many other large corporations that have helped support the Chamber’s pro-business advocacy and ubiquitous political issue ads remain secret to the public.

Just over two dozen publicly traded companies have disclosed their donations to the organization — totaling about $11 million.

The Chamber — one of Washington’s major political powers, with annual revenue of $200 million — pledged to spend $100 million this election cycle to support candidates focused on corporate concerns. But the corporate trade association does not have to identify the companies that donate, and those firms can decide whether to share that information.

“We’re pleased to report this, but not every company provides that level of disclosure,” said Bob DeFillippo, a spokesman for Prudential, which donated more than $1.5 million to the organization. “It’s important to see you’re not getting the full picture. But for us, it’s a matter of being transparent.”

Donating to the Chamber can be a double-edged sword for some corporations with a broad consumer base. Although the organization’s pro-business advocacy can benefit companies, the chamber has a controversial and generally pro-Republican agenda, which could anger some customers. Apple, Nike and other companies loudly quit the Chamber in 2009 because it opposed policies to combat climate change.

“What is true is that publicly traded companies are very worried about their shareholders, the public and their customers knowing that they’re giving this money,” said Public Citizen President Robert Weissman. “What we see is that when it isn’t disclosed, companies are more interested in giving.”

The information about donations in 2011 came from voluntary corporate disclosures, recently compiled by Public Citizen and the Center for Political Accountability, which support more mandatory disclosure of donations.

Chamber leaders said its foes seek to make donors wary by creating a controversy. They say their campaigns aim to educate voters about policies that can boost the economy and create jobs.

“The fact that a particular company is a member of the Chamber is not controversial to the vast majority of the public,” Chamber spokeswoman Blair Latoff said. “We’re not interested in facilitating harassment and intimidation of individuals and businesses for doing nothing more than engaging in protected speech and exercising their First Amendment freedom of association.”

Advocacy groups — including U.S. PIRG, Public Citizen, the Business Ethics Network and Citizens for Responsibility and Ethics in Washington — are calling for more mandatory disclosure.

“Corporations should operate on the premise that their political donations will be disclosed,” said Bruce Freed of the Center for Political Accountability. “Secrecy breeds abuse and undermines accountability.”

The pro-disclosure groups will mark the 100th birthday of the chamber at 11 a.m. Friday with a protest and rally outside the chamber’s grand Beaux Arts headquarters facing the White House.

The disclosure advocates ultimately want a constitutional amendment to overturn a pivotal 2010 Supreme Court ruling, Citizens United v. Federal Election Commission, which allowed unrestricted corporate political spending.

In 2010, the Chamber reported total political expenditures of $32 million. It has spent $27 million in the 2012 cycle.

Companies that disclose their donations do not have to specify which portion is for dues or special projects, or how much could be used for political lobbying and ads. Dow said it gave the Chamber a total of $1.6 million, without detailing which portion could be used for lobbying. Merck reported that last year it donated $725,000 that could be used for political lobbying, but it didn’t specify a total contribution.

Prudential told shareholders that it gave the Chamber $570,000, about half of that for lobbying. It also reported giving an additional $1 million to the Chamber’s Institute for Legal Reform, which seeks tort reform and liability limits, and said more than two-thirds of that was for political lobbying.

Companies such as Coca-Cola, Chubb Insurance, MetLife, computer-chip-maker Intel and defense contractor Lockheed Martin also shared information about their donations, the disclosures show.

The Chamber gives companies an estimate of the proportion of their donations — which can be as much as half — used for lobbying purposes, including political ads.

Prudential’s DeFillippo said the company has an agreement with the Chamber that its contributions will be used only for “operational purposes” and not political campaigns.

Intel said its $190,000 contribution goes to pay dues and to contribute to the Chamber’s special program to increase market access for U.S. corporations in foreign countries. Intel trade policy director Greg Slater said the Chamber has helped resist protectionist trade practices in some countries.

Several corporate spokespeople said their donations were logistical support for a trade group that advances their shareholders’ and clients’ interests, whether on tort reform or health-care legislation.

“Dow participates in many trade and business associations, including the U.S. Chamber of Commerce, to assist in managing our policy priorities important to businesses, including tax reform, energy, among other issues,” company spokeswoman Nancy Lamb said.

MetLife gives these donations, spokesman John Calagna said, because it “along with virtually every major company and thousands of small businesses . . . respects the Chamber for its strong voice on behalf of business as the key to a strong economy and country.”

Alice Crites, Dan Eggen, T.W. Farnam and Tom Hamburger contributed to this report.