U.S. Environmental Protection Agency chief Lisa Jackson, who spearheaded the Obama administration's crackdown on carbon emissions, said on Thursday that she will be stepping down after almost four years of battles with Republicans and big industry over her proposed regulations.

Under her leadership, the agency declared for the first time that carbon dioxide was a danger to human health and could be regulated under the Clean Air Act, leading the EPA to develop a new regulatory regime to limit carbon emissions.

Leading the list of potential replacements at the agency are Bob Perciasepe, the current deputy EPA administrator, who will take over the agency on an interim basis; and Kathleen McGinty, a former head of Pennsylvania's Department of Environmental Protection and a protege of former U.S. Vice President Al Gore.

Jackson's departure was not a surprise. Observers had not expected her to stay for President Barack Obama's second term.

She had faced withering attacks from industry groups and Republicans, who hauled her in for numerous hearings in Congress, as well as some pushback from within the Obama administration.

Obama thanked Jackson for her service, praising her work on mercury pollution limits, fighting climate change and helping set new fuel economy standards for vehicles.

"Under her leadership, the EPA has taken sensible and important steps to protect the air we breathe and the water we drink," President Barack Obama said in a statement.

Jackson, 50, is expected to depart the EPA after Obama's State of the Union address in early 2013. The post of EPA administrator is a Cabinet-level job.

BRUISING ENCOUNTERS

Her time at the agency was marked by some bruising encounters with Republican lawmakers, who accused her of orchestrating massive government overreach that they said was choking U.S. economic growth.

For many Republicans, the EPA under Jackson was responsible for a radical government intrusion into the free economy.

Republican lawmakers passed numerous bills aimed at undoing Jackson's environmental regulations. None of these measures were signed into law, but the White House did begin to pull back or delay rules in the face of the relentless onslaught.

For example, Obama's decision in 2011 to delay rules to restrict emissions of smog-forming chemicals from power plants led to speculation that Jackson would step down at that time in response to the setback.

In general, measures to address climate change by the Obama administration have been much more timid than what many environmental activists had wanted.

In a statement, Jackson, the first black administrator of the 17,000-strong EPA, said she was "confident the (EPA) ship is sailing in the right direction."

"Lisa Jackson has shown an unwavering commitment to the health of our families and our children," Obama said, adding that her shepherding of new fuel economy standards "will save the average American family thousands of dollars at the pump."

Reports in recent weeks have suggested that Jackson, a chemical engineer by training, might be under consideration for the post of president of Princeton University in New Jersey.

Media reports also indicate that Jackson, who was a chief of staff to former New Jersey Governor Jon Corzine, might be mulling a run for governor in that state.