In a previously unreported May 18 letter to Treasury Secretary Timothy Geithner, the leaders of the American Federation of State, County and Municipal Employees and the AFL-CIO called for the resignation of Michael Armstrong and John Deutch as directors of Citigroup.

“In our view, these directors had catastrophically failed to protect shareholders from excessive exposure to credit, market, liquidity and operational risk,” wrote AFSCME President Gerald McEntee and AFL-CIO President John Sweeney. “Companies like Citigroup that have received taxpayer assistance under the Troubled Asset Relief Program should be held to the highest corporate governance standards.”

The union leaders have not yet received a reply from Treasury.

Both unions endorsed Obama in June 2008, and their members campaigned hard for him in the general election.

The union-led campaign against corporate directors already has claimed the job of American International Group board member James F. Orr III, chairman of the Rockefeller Foundation. He decided not to stand for reelection in late May, along with two other AIG board members. McEntee was among the shareholder activists who sent a March 31 letter to government trustees blasting Orr for his service on the AIG board’s compensation committee at the time it approved millions of dollars in controversial bonuses to some of the very AIG employees whose bad market bets brought the company down.

In the latest case, the union leaders say they object to the April 21 reelection of Armstrong and Deutch because the two men received 1.1 billion votes against their election. And although they received 2.7 billion votes for reelection, the union claims that 1.7 billion of those votes were cast by stockbrokers on behalf of shareholders who otherwise would not have voted their shares. So-called broker voting is allowed under corporate governance rules, but the unions complain that the brokers vote in lock step with management and that elections in which they cast ballots on behalf of their clients are not a true reflection of shareholder intent.

For its part, Citigroup defends the targeted board members.

“Citi’s board of directors have diligently carried out their responsibilities during one of the most severe market downturns in decades,” said spokeswoman Molly Millerwise Meiners. “The board also rotated committee chairs and members in July 2008. There is no basis for any recommendations against directors.”

Armstrong is the former CEO of AT&T, and Deutch was director of the CIA during the Clinton administration. During his tenure as the nation’s top spy, Deutch was caught up in a controversy over allegations that he had kept classified materials on unsecure home computers. President Bill Clinton pardoned him on his last day in office.

The heightened union aggressiveness against directors has raised eyebrows across corporate America.

“What is going on here is a concerted effort to push an activist agenda,” said Tom Quaadman, executive director of the Center for Capital Markets Competitiveness at the U.S. Chamber of Commerce. “Should 150 years of corporate governance law be thrown out to satisfy political agendas? We think that’s wrong.”

The anti-director push represents a newly emboldened union movement — bolstered by allies in the White House and a vast federal intervention in the private sector — that is eager to put its stamp on the way American business is run. It comes at a time when some corporate bondholders are complaining that the United Auto Workers, through its influence inside the Obama administration, got the best part of the GM bankruptcy deal announced by Obama on Monday. The White House insisted that deal was equally tough on all parties.

“We want all directors to be concerned that we will go after them, because that’s the way we will keep them accountable,” said Richard Ferlauto, the director of corporate governance and pension investment for AFSCME and a leader of the anti-director effort. “We are pretty good at identifying directors who we think are not up to the job.”

Ferlauto said his coalition will target additional directors once the Citigroup situation is resolved. Union pension funds are shareholders in the companies in question, he added, and have every right to agitate for better results from the companies.

“The, quote, ‘muscle’ here isn’t union muscle; it’s ownership muscle,” Ferlauto said. “As owners, we have every incentive to make the companies thrive and prosper.”