CARSON CITY, Nev. - U.S. Senate Majority Leader Harry Reid, stepping up efforts to block new coal-fired power plants in Nevada, is introducing a bill requiring most power transmitted over a proposed cross-state line to come from alternative energy sources.

Reid's measure, to be introduced Thursday in the U.S. Senate, would require Nevada-based Sierra Pacific Resources to ensure that 75 percent of the power transmitted over its planned high-voltage line comes from solar, wind, geothermal or other alternative energy sources.

If Sierra, parent of Reno-based Sierra Pacific Power Co. and Las Vegas-based Nevada Power, didn't build the line, the bill by Reid, D-Nev., gives bonding authority to the Western Area Power Administration so it can do the job.

Sierra has estimated cost of the big power transmission line at nearly $600 million. WAPA would have bonding authority of up to $10 billion, under Reid's measure.

"Sen. Reid is very clear in his opposition to the coal-fired power plants but also realizes the need to replace them with something else," spokesman Jon Summers said Wednesday.

If Sierra decides not to build the line, Summers said WAPA could construct and control it "while at the same time promoting the renewable energy industry."

Reid recently came out against three coal-fired power plants proposed for eastern Nevada. Two are near Ely and the third is near Mesquite. Reid said the Ely-area projects alone would require millions of tons of coal a year that in turn would generate millions of tons of pollution.

Sierra Pacific, Sithe Global Power LLC in New York, LS Power in New Jersey and Dynegy Inc. in Texas are proposing the plants. Sierra Pacific and Sithe Global have separate projects while LS Power and Dynegy are developing the third project.

Reid's stance has drawn praise from environmental groups while representatives of the companies planning the projects defend them as necessary given scheduled shutdowns of older plants and the explosive growth in the Southwest, especially in Las Vegas.

While Reid is fighting the coal-fired plants, the Nevada Environmental Commission has rejected a petition from seven environmental groups trying to stop the companies from constructing such plants unless they control carbon dioxide emissions.

Gov. Jim Gibbons has opposed the move by the environmental groups, saying it isn't fair to stop the plants when they already "have invested years and millions of dollars in facility design and the permitting process."