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By Timothy Gardner

NEW YORK, July 20 (Reuters) - An across-the-board downgrading of U.S. coal company stocks by a Citigroup Inc. <C.N> analyst is the latest victory in a fight against plans for new coal-fired power plants, environmentalists said.

Citigroup analyst John Hill downgraded coal company stocks across the board in a report this week, saying that expected U.S. greenhouse gas regulations on coal, which emits more of the main heat-trapping gas carbon dioxide than any other fuel, paint a bleak outlook for the sector.

Downward pressure on stock prices by a current U.S. coal oversupply could last for more than a year, he wrote. If that happens it could coincide with 2008 presidential campaign politics, in which a national plan to limit greenhouse emissions is expected to figure prominently.

"Election politics are likely to turn progressively more bestial for coal," Hill wrote. Candidates from both major parties favor putting national limits on greenhouse gases.

Environmentalists, who have recently helped block plans for coal plants, seized on the comments as an indication of harder times ahead for companies that produce the fuel.

"This is a clear, forceful signal from Wall Street that the coal industry is failing to innovate in addressing the urgent problem of global warming," Vicky Patton, a lawyer for Environmental Defense, wrote in an e-mail.

The Department of Energy says utilities plan to build about 150 coal-fired power plants. But coal companies have recently suffered a string of bad news from the top three most populous U.S. states.

California passed a global-warming law halting the building of coal-fired plants, as well as a ban on imports of power generated in other states that does not meet emissions standards.

Then in Texas, TXU Corp. <TXU.N> canceled plans for eight new coal-fired plants as part of an agreement to be bought by private equity companies that were advised by Environmental Defense.

Last week, Florida Gov. Charlie Crist signed one of the country's toughest caps on greenhouse emissions, soon after plans there for two coal-powered stations had been scotched.

And in New York, the country's fourth most-populous state, CO2 caps on coal plants are slated for 2009 as part of a 10-state regional pact in the Northeast to cut emissions. The pact effectively diminishes the likelihood that new coal-fired generation will be built in the 10 states.

"Prophesies of a new wave of coal-fired generation have vaporized," Hill wrote. And technology to capture and bury CO2 at power plants may play a role in coming years, but remains expensive and elusive.

To be sure, there are some factors that could support coal which currently generated half of U.S. power, including tougher safety laws that could shut costlier coal production in the East, said analyst Jeremy Sussman at Natexis Bleichroeder in New York.

Coal companies in the U.S. East could increasingly become exporters to the international market. "Coal that used to go to Europe is now going to China, so that leaves a void for U.S. producers to fill," he said.

Others said anti-coal laws on the coasts could simply mean more coal burned in other regions of the country, at least until the country adopts national CO2 caps.

But there is an effort afoot to stop coal plants in those places, too. Trip Van Noppen, vice president at environmental law firm Earthjustice, said his group is fighting power plants in Kansas, Montana and New Mexico.

Top coal shares are down about 10 percent from their year highs hit early in 2007. Peabody Energy <BTU.N> shares were down 2.47 percent to $46.21, Arch Coal <ACI.N> shares were down 1.36 percent to $33.44, and Massey Energy <MEE.N> shares were down 3.78 percent to $23.45.

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