With a staunchly anti-environmental White House and a Republican-dominated Congress, environmentalists are turning to the third branch of government to fight their cause. Happily, the courts have presented a relatively safe haven for greens, upholding strict clean air standards the Bush administration sought to water down, blocking oil and gas exploration in the West, limiting mountaintop mining and dumping in Appalachia, and — in the biggest victory for environmentalists — protecting millions of acres of national forest from logging and road building. “The courts are being viewed as the last line of defense,” said Buck Parker, executive director of Earthjustice, a public-interest environmental law firm.