Fellow board members and company executives, who have not spoken publicly about her role at Wal-Mart, say Mrs. Clinton used her position to champion personal causes, like the need for more women in management and a comprehensive environmental program, despite being Wal-Mart’s only female director, the youngest and arguably the least experienced in business.

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And several months ago [in 2007], Mrs. Clinton helped broker a secret meeting between a top Wal-Mart executive and former Democratic operative, Leslie Dach, and leaders of the retailer’s longtime adversary at the United Food and Commercial Workers union, according to several people briefed on the matter, who spoke on condition of anonymity because they were not authorized to do so publicly.

The goal of the meeting was to tamp down the rancor between the company and the union, which has set up a group, WakeUpWalMart.com, that has harshly criticized the chain and leaked embarrassing internal documents to the news media, though an accord has not yet been reached.

Mrs. Clinton declined to be interviewed for this article. In a statement, her spokesman said, “Wal-Mart is now one of the country’s largest employers, and Mrs. Clinton still believes it is important to try to influence the decisions they make because they can affect so many people.”

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“Did Hillary like all of Wal-Mart practices? No,” said Garry Mauro, a longtime friend and supporter of the Clintons who sat on the Wal-Mart Environmental Advisory Board with Mrs. Clinton in the late 1980s and worked with her on George McGovern’s 1972 presidential campaign.

“But,” Mr. Mauro added, “was Wal-Mart a better company, with better practices, because Hillary was on the board? Yes.”