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Less than two weeks after coming out against the Keystone XL pipeline project, which environmental groups have denounced but many labor unions support, Hillary Rodham Clinton on Friday criticized a coal company’s bankruptcy plan as detrimental to the families of the miners who work for the company.

The comment, made in a statement to the Reuters news service, came as Mrs. Clinton is facing opposition within some major labor unions. It also came as Senator Bernie Sanders, the independent from Vermont, makes gains in polls, and as Vice President Joseph R. Biden Jr., a favorite of some labor groups, considers joining the 2016 presidential race.

A plan that Patriot Coal Corporation has proposed as a bankruptcy settlement is “outrageous and must be stopped,” Mrs. Clinton said in the statement.

“Patriot Coal is trying to take $18 million of the $22 million put aside for retired coal miners, wives and widows and use it to pay its lawyers instead,” she said. “Ensuring health care and retirement security should be the first priority in a bankruptcy proceeding, not the last.”

At issue is a settlement involving an Indiana mine.

Mrs. Clinton’s statement could appeal to working-class voters. It was released on the same evening that her husband, former President Bill Clinton, was the keynote speaker at a Democratic dinner in West Virginia, a state that has little chance of being in play in a general election but which is heavy with working-class white voters.

Earlier this week, Mrs. Clinton said she opposed the Affordable Care Act’s so-called Cadillac tax, reviled by labor groups, which will apply to most coverage under health care packages whose premiums exceed certain amounts. Officials with the American Federation of Teachers, the largest union to endorse Mrs. Clinton so far, had pressed her to take such a position.

On Friday, officials with the International Association of Fire Fighters said the union was backing away from its plan to endorse Mrs. Clinton, in part because of hopes that Mr. Biden will run. The union supported Senator Christopher J. Dodd of Connecticut in the 2008 Democratic presidential primary, and it did little to help him move ahead in the contest. But amid Mrs. Clinton’s recent slide in the polls, the union’s decision adds to her difficulties.

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