EPA Administrator Lisa Jackson speaks at the 2012 Tribal Nations Conference earlier in December.Jackson, who pushed through the most sweeping curbs on air pollution in two decades, announced Dec. 27, 2012 that she will resign her post. (Susan Walsh/AP)

Environmental Protection Agency Administrator Lisa P. Jackson, who pushed through the most sweeping curbs on air pollution in two decades, announced Thursday morning that she will resign her post.

Jackson, who will step down shortly after President Obama’s State of the Union address next month, said she was “ready in my own life for new challenges, time with my family and new opportunities to make a difference.” Many had expected that she would not remain for the administration’s second term; Jackson herself joked about it recently.

Outspoken on issues including climate change and the need to protect poor communities from experiencing a disproportionate amount of environmental harm, Jackson pressed for limits on emissions from coal-fired power plants and on dumping mining waste into streams and rivers near mines.

The slew of rules the EPA enacted over the past four years included the first greenhouse-gas standards for vehicles, cuts in mercury and other toxic pollution from power plants and a tighter limit on soot, the nation’s most widespread deadly pollutant. Many congressional Republicans and business groups claimed Jackson was waging a “war on coal.” But she was a hero to the environmental community.

The president issued a statement praising Jackson.

“Under her leadership, the EPA has taken sensible and important steps to protect the air we breathe and the water we drink, including implementing the first national standard for harmful mercury pollution, taking important action to combat climate change under the Clean Air Act, and playing a key role in establishing historic fuel economy standards that will save the average American family thousands of dollars at the pump while also slashing carbon pollution,” Obama said.

Obama has not picked her successor, although two of the leading candidates work at the EPA: Deputy Administrator Bob Perciasepe and Gina McCarthy, who heads the agency’s air and radiation office. Jackson has told several people she considers Perciasepe well prepared to take the agency’s helm.

Both Perciasepe and McCarthy could face challenges in getting confirmed by the Senate since they helped craft many of the EPA’s policies during Obama’s first term. But they are also seen as career officials rather than political activists. Stephen Brown, vice president for federal government affairs at the oil refiner Tesoro Corp., said Perciasepe represents “a sound choice” because he “knows the job, the agency and the hurdles associated with both.”

Other possible successors include Mary D. Nichols, head of the California Air Resources Board, and Kathleen McGinty, who headed the White House Council on Environmental Quality under President Bill Clinton.

Sen. David Vitter (La.), the top Republican on the Senate Environment and Public Works Committee, said Thursday he would seek a more business-friendly nominee to run the agency. “Moving forward I’ll be working with my colleagues in the Senate to make sure the new nominee is thoroughly vetted, puts sound scientific standards above political ideology and understands that EPA’s avalanche of regulations can crush the growth of American businesses,” he said in a statement.

It remains unclear how ambitious an agenda the EPA will pursue in Obama’s second term. The agency will soon finalize the first carbon standard for new power utilities, but the White House has yet to decide whether to impose limits on existing facilities, according to several individuals who have been briefed on the matter but asked not to be identified because no final decision has been made.

The EPA has delayed a decision on whether to classify coal ash as a hazardous waste and is fighting in federal court to reinstate rules governing cross-state air pollution from coal plants in the eastern half of the country. It is also in the midst of a long-term study of how hydraulic fracturing affects the environment, which could trigger new federal rules governing natural-gas extraction.

Michael Brune, executive director of the Sierra Club, said his members will be looking for the EPA to “have a strong voice” in whether Obama should approve the Keystone XL pipeline carrying heavy crude oil from Canada to the United States, and to press ahead with carbon limits on existing power plants. “It’s arguably the biggest thing the administration can do by itself, without legislation, to reduce greenhouse-gas emissions,” he said in an interview.

Many of the most significant regulations the EPA has enacted over the past four years arose from settlements with environmental groups, which challenged rules the agency issued under President George W. Bush.

“Most of the significant rules have already been finalized, so I’m not sure what’s left,” said Joseph Stanko, who heads government relations at the law firm Hunton & Williams and represents several utility companies.

While Jackson successfully pushed for a number of landmark initiatives, including limits on nutrient pollution flowing from several states into the Chesapeake Bay, she also suffered a high-profile setback when Obama pulled an EPA proposal last year to curb smog-forming ozone pollution. At the time, the president said the new rules would unnecessarily damage the economy and were not essential because the agency was slated to review the issue in 2013.

The child of a postal worker who grew up in the 9th Ward of New Orleans, Jackson often spoke of her personal history when explaining her public policy decisions. When discussing climate change and environmental disasters, she recalled driving her mother, stepfather and aunt out of the city in the face of Hurricane Katrina, which destroyed her mother’s home; when announcing new air quality rules, she frequently referred to the agony she felt as a mother watching her infant son struggle with asthma. During an interview with The Washington Post, she once started singing Stevie Wonder’s 1973 hit “Living for the City” to describe how far the country had come in terms of cleaning up air pollution.

While she charmed some of her critics — Sen. James M. Inhofe (R-Okla.) referred to Jackson as “my favorite bureaucrat” — she alienated much of the business community. Ross Eisenberg, vice president for energy and resources policy at the National Association of Manufacturers, complained to reporters recently that Jackson and her deputies consistently failed to achieve a balance when regulating pollutants from industrial activities.

“EPA seems wedded to the notion that it must push its policies as hard as it possibly can, with the goal being to enact the strictest possible standard that will survive legal scrutiny,” Eisenberg said. “That’s not EPA’s job.”

Scott Segal, a partner at the law firm Bracewell & Giuliani who represents coal-fired utilities as director of the Electric Reliability Coordinating Council, wrote in an e-mail that Jackson managed to drive environmental policy in Washington because of “her excellent communications skills, likable personality and skilled use of political leverage.”

But Segal also criticized her for enacting costly regulations and misleading the public by exaggerating their benefits: “Agency rules have been used as blunt attempts to marginalize coal and other solid fossil fuels and to make motor fuels more costly at the expense of industrial jobs, energy security and economic recovery.”

Jackson said in an interview that she is open to pursuing consulting and public speaking and misses New Jersey, which is where she and her family lived before moving to Washington in 2009. Her name has been floated as a possible candidate for the presidency of Princeton University, where she received a graduate degree in engineering.