The California Public Employees Retirement System (Calpers) is once again flexing its proxy muscle, this time vowing not to vote for re-election of Citigroup ( C) chairman Sanford Weill and chief executive Charles Prince to the bank's board.

Calpers, which owns 26.7 million shares of Citigroup stock, says it is opposing Weill because he had a "significant role in several scandals" that hurt the image of the world's largest financial services firm.

"We believe director Weill should be held accountable for substantial costs incurred by settling civil investigations of the company's improper practices and conflicts of interest," Calpers said in a statement posted on its Web site Monday.

Weill stepped down as chief executive of Citigroup last year. He is credited with turning Citigroup into one of the most powerful and profitable financial firms in the world.

But the end of his tenure was marred by the bank's role in providing questionable financing to Enron, WorldCom and other scandal-tainted companies. Additionally, Citigroup ponied up a huge fine in last year's $1.4 billion Wall Street conflicted research settlement.

Calpers is also opposing the election of Prince, the man tapped to succeed Weill as chief executive. The pension fund says Prince's business relationships could "impair" his objectivity as a board member.

The pension also is opposing the election of six other Citigroup board members: Michael Armstrong, Alain Belda, George David, Kenneth Derr, John Deutch and Roberto Hernandez.

In all, 16 board members are on the ballot. Citigroup has scheduled its shareholder meeting for April 20.

A Citigroup spokeswoman could not be reached for comment.

In recent years, Calpers has taken a more activist footing in opposing the election of board directors.