Health insurance giant WellPoint is the latest target of an increasingly aggressive campaign to force disclosure of corporate political and lobbying expenditures, including payments to the U.S. Chamber of Commerce, which has become more active in elections over the past decade.

The WellPoint campaign, set to be formally announced Thursday by a coalition of activist investor groups, demands the resignation of two WellPoint board members, including Susan Bayh, the wife of former senator Evan Bayh (D-Ind.), for allegedly failing to oversee “high risk political spending.”

The shareholder coalition cited WellPoint’s reluctance to answer questions about a transfer of $86 million from the health insurers trade association to the U.S. Chamber of Commerce in 2010, when the Chamber was actively opposing President Obama’s health-care overhaul. WellPoint is a member of the association, America’s Health Insurance Plans.

“This is the most egregious clandestine campaign funding we have ever seen,” said Michael Pryce-Jones of the CtW Investment Group, a labor-affiliated organization that is part of the shareholders’ coalition, referring to the payments from the trade association to the Chamber of Commerce.

The coalition’s effort to hold specific corporate board members responsible represents a new militancy in the fight to require companies to reveal their political activities. It also foreshadows concern about the potential for heavy spending by industry and labor groups on health care and other issues in the 2012 election.

Officials at WellPoint and the Chamber of Commerce rejected the investor group’s allegations on Wednesday.

“The target here is not a specific company but the ability of all companies to participate in public policy debates,” said Thomas J. Collamore, senior vice president for communications and strategy at the Chamber. “This is part of a coordinated effort by the left to silence the business community and create one-sided debates, the only ones they can win.”

At WellPoint, officials dismissed the notion that the company has been secretive about its political giving. On the contrary, spokeswoman Kristin Binns said, the firm discloses a great deal on its Web site.

“WellPoint complies with all disclosure requirements under federal, state and local laws,” she said, noting that the company publishes a “very extensive” annual report on its political contributions.

That report does not include details of the sort of special payment that the shareholders coalition said WellPoint made to the health insurers association.

Shareholder resolutions demanding disclosure and other reforms have proliferated in recent years. But corporate governance advocates say this seems to be the first effort to target specific board members for decisions about spending.

“This is a surgical strike, and it could be enormously effective,” said Nell Minow, director of GMI Ratings, which provides corporate governance information to auditors and investors. “Targeting individual board members may be the only way you make any progress” in forcing the disclosure of corporate political and lobbying activities.

Minow joins a growing group of corporate governance experts urging more disclosure because of the “reputational risks” for companies when controversial spending is revealed.

The WellPoint campaign is part of an increasingly vitriolic war between Democrats and their labor allies on the one hand and Republicans and business-oriented groups on the other. Independent organizations advocating more rigorous corporate governance standards find themselves caught in the dispute over disclosure.

The pressure on corporations to disclose political and lobbying activities spiked after the 2010 Supreme Court decision in Citizens United v. Federal Election Commission, which permitted companies to contribute directly to political groups.

The danger to corporations was evident in the past two weeks as activists pressed a dozen firms to cut their financial support for a little-known but effective conservative lobbying group, the American Legislative Exchange Council. The group focuses on state legislative issues, including promoting “stand your ground” laws, which gained national attention after the fatal shooting of 17-year-old Trayvon Martin in Florida.

After Martin’s death, activists targeted companies with memberships in ALEC, asking them to sever ties to the group. Several firms, including Coca-Cola, Pepsi, McDonald’s and Intuit, announced that they would not renew their memberships. Binns, WellPoint’s spokesperson, said the company has not participated in ALEC this year.

Most shareholder and boycott efforts are less successful. Binns noted that a shareholder resolution two years ago asking WellPoint to provide more disclosure was defeated by 82 percent of voters.

On the other hand, a growing number of major companies have adopted rules for disclosure and oversight of their political spending. Currently, 96 firms in the S&P 500 have committed to disclosure and oversight, according to a study by the Center for Political Accountability. The tactic of holding individual board members to account may add fuel to the overall effort at a time when shareholder resolutions are having some success.

On Tuesday, 55 percent of Citigroup’s shareholders supported a resolution at the company’s annual meeting opposing the pay packages offered to top executives.

In a letter that is set to be mailed Thursday to WellPoint shareholders, the investors’ coalition urged opposition to the two board members as a way of holding them responsible for governance decisions.

“These directors must be held accountable for WellPoint’s refusal . . . to disclose the details of its high risk political spending,” the coalition said in the letter.