WASHINGTON – EPA Administrator Lisa Jackson, the Obama administration’s chief environmental watchdog, is stepping down after nearly four years marked by high-profile brawls over global warming pollution, the Keystone XL oil pipeline, new controls on coal-fired plants and several other hot-button issues that affect the nation’s economy and people’s health.

Jackson constantly found herself caught between administration pledges to solve thorny environmental problems and steady resistance from Republicans and industrial groups who complained that the agency’s rules destroyed jobs and made it harder for American companies to compete internationally.

The GOP chairman of the House Energy and Commerce Committee, Rep. Fred Upton of Michigan, said last year that Jackson would need her own parking spot at the Capitol because he planned to bring her in so frequently for questioning. Republican presidential nominee Mitt Romney called for her firing, a stance that had little downside during the GOP primary.

Jackson, 50, the agency’s first black administrator and a chemical engineer, did not point to any particular reason for her departure. Historically, Cabinet members looking to move on will leave at the beginning of a president’s second term.

Despite the opposition, Jackson still managed to take significant steps that are aimed at improving air quality and curbing global warming.

“I will leave the EPA confident the ship is sailing in the right direction, and ready in my own life for new challenges, time with my family and new opportunities to make a difference,” she said in a statement. Jackson will leave sometime after President Barack Obama delivers his State of the Union address, typically in late January.

In a separate statement, Obama said Jackson has been “an important part of my team.” He thanked her for serving and praised her “unwavering commitment.”

Environmental activist groups and other supporters lauded Jackson for the changes she was able to make, but industry representatives said some may have come at an economic cost.

Groups also noted that she leaves a large, unfinished agenda.

“There has been no fiercer champion of our health and our environment than Lisa Jackson, and every American is better off today than when she took office nearly four years ago,” said Frances Beinecke, president of the Natural Resources Defense Council.