By Valerie Volcovici

WASHINGTON, March 3 (Reuters) - The U.S. Supreme Court sided with the Obama administration on Thursday in rebuffing a bid by 20 states to halt an Environmental Protection Agency rule to curb emissions of mercury and other toxic pollutants from power plants.

The action came about a month after the high court put on hold federal regulations to curb carbon dioxide emissions mainly from coal-fired power plants, the centerpiece of President Barack Obama's strategy to combat climate change.

Chief Justice John Roberts denied a petition made last week by the states, led by Michigan, to put the rule on hold after a federal appeals court decided in December to leave it intact while the EPA reassessed costs of implementing the regulation.

The states had argued that a stay was necessary because the Supreme Court had "already held that the finding on which the rule rests is unlawful and beyond the EPA's statutory authority."

The Supreme Court ruled last June that the EPA should have considered compliance costs when it decided to limit emissions of mercury and other hazardous air pollutants mainly from coal-fired power plants, and returned the case to the appeals court.

The U.S. Court of Appeals for the District of Columbia Circuit subsequently said the regulations could remain in place while the government responds to the high court's June ruling. The appeals court gave the EPA until April 15 to come up with compliance costs.

The EPA rule, which went into effect last April, applies to about 1,400 electricity-generating units at 600 power plants. Many are already in compliance.

EPA spokeswoman Melissa Harrison said the agency was "very pleased" with Roberts' action, noting that power plants are the largest source of mercury pollution in the United States.

"These practical and achievable standards cut harmful pollution from power plants, saving thousands of lives each year and preventing heart and asthma attacks," Harrison said.

Environmental groups and 15 other states that had asked the court to reject the stay request applauded Roberts' action and called on the attorneys general in the group of states led by Michigan to end their legal challenge to the rule.

The rule is "safeguarding millions of American children from poisons such as mercury, arsenic and acid gases," said Vickie Patton, general counsel for the Environmental Defense Fund anti-pollution advocacy group, a party to the case. (Reporting by Valerie Volcovici; Editing by Will Dunham)