The ozone almost did her in.

Lisa Jackson, the Environmental Protection Agency administrator, was on the verge of quitting two weeks ago after President Obama rejected her centerpiece proposal for strict new air-pollution regulations, The Post has learned.

Jackson told intimates she had been convinced Obama would back her up despite aggressive lobbying from Capitol Hill Republicans and business interests.

But on Sept. 1, she learned her support in the West Wing had crumbled during two meetings — one with Chief of Staff Bill Daley and another with the president.

In the face of a weak economy, bad poll numbers and bleak employment figures, Obama made it clear that “we just don’t need this fight right now,” according to an administration source.

A day later, the administration announced it was abandoning the proposal for the tougher emissions rules, which would have cut the amount of ozone that can be spewed into the air.

The White House said that it would revisit the issue in 2013, but that, for now, the changes to the Clean Air Act would have burdened businesses and local governments with costs to contain and monitor air pollution.

“She was very upset,” one administration official told The Post of Jackson’s reaction. “She didn’t know what she was going to do.”

Sources said Jackson spent two days considering whether to quit.

“Lisa was blindsided,” said senior Sierra Club official Jeff Tittel, a longtime confidant of Jackson. Obama “did it and then told her about it. She was pretty pissed off. She felt like she got hit with a hot poker between the eyes.”

But by the time she traveled with Obama to view hurricane damage on Sept. 4, Jackson had cooled and was telling intimates she was determined to stay.

Tittel said Jackson, 49, “has a lot of other stuff out there that she wants to get done that wouldn’t get done if she left. She’s not the kind of person who goes home and takes her marbles. She’s the kind who would stay and fight even if she is frustrated.”

The White House declined to comment.

Jackson spokesman Brendan Gilfillan said, “This administration has a tremendous record on the environment and a lot more work left to do.

“Administrator Jackson said she’s not going anywhere, and she isn’t.”