WASHINGTON — The Obama administration is proposing a new rule to tighten restrictions on pollution from coal-burning power plants in the eastern half of the country, a key step to cut emissions that cause smog.

The Environmental Protection Agency said Tuesday the new rule represented its most consequential effort yet to tackle deadly pollution that contributes to smog and soot that hangs over more than half the country.

The rule would cost nearly $3 billion a year and those costs are likely to be passed along to consumers, although the rule’s effect on specific companies and on consumers was not clear.

“We believe that today is marking a large and important step in EPA’s effort to protect public health,” said the agency’s top air pollution official, Gina McCarthy.

The rule, to be finalized next year, aims to cut sulfur dioxide emissions by 71 percent from 2005 levels by 2014 and nitrogen oxide emissions by 52 percent in the same time frame.

Known as the Clean Air Interstate Rule, the measure requires 31 states from Massachusetts to Texas to reduce smog and soot-producing emissions that can travel long distances in the wind.

The rule would overturn and toughen rules issued during the administration of former President George W. Bush.

While environmental groups and some Democratic lawmakers hailed the new regulation, they conceded the complicated measure is open to industry lawsuits that could cause delays in meeting public health targets.

Sen. Tom Carper, D-Del., said the likelihood of litigation underscores the need for Congress to pass strong air pollution legislation this year.

With a comprehensive energy bill facing united GOP opposition in the Senate, Democrats are considering an approach that would focus on capping greenhouse gas emissions from power plants. Some White House officials have begun to speak favorably about such a “utility-only” bill.

Frank O’Donnell, chairman of Clean Air Watch, cautioned Democrats not to sacrifice the interstate pollution rule to win votes for a climate bill.

More than a dozen states, along with environmental groups, sued the EPA several years ago, contending the Bush administration ignored science when it decided in 2006 not to lower the soot standard.

EPA Administrator Lisa Jackson said the rule signed Tuesday should improve air quality and public health from southern New England down to Florida, over to Texas and up to Minnesota. The rule does not affect Maine, Rhode Island, Vermont and New Hampshire.