"We do not support all of the positions taken by organizations to which we belong," Occidental's associate general counsel, Linda Peterson, wrote.
Occidental's departure from ALEC comes after several large tech companies left the organization earlier this month. Google, Yahoo, and Yelp publicly announced they have already or are currently divorcing themselves from the coalition. Facebook also stated it was "not likely" to renew its membership with the organization in 2015.
Last week, Google Chairman Eric Schmidt started the mass exodus by condemning ALEC for "just literally lying" about climate change.
"We funded them as part of a political [campaign] of something unrelated," Schmidt told NPR. "I think the consensus within the company was that that was some sort of mistake, and so we're trying to not do that in the future."
Occidental is one of the largest U.S.-based international oil and gas exploration and production companies. The company has operations in the U.S., the Middle East, and South America. It is the largest natural-gas producer in California and the largest oil producer in the Permian Basin, a major oil- and gas-producing region that spans parts of Texas and New Mexico.
The company also has a track record of supporting conservative candidates. In the current election cycle, 93 percent of all spending to back federal candidates from Occidental Petroleum's PAC has gone to Republicans. That adds up to a total of approximately $255,750, according to the Center for Responsive Politics. Occidental's PAC has backed Republicans over Democrats by wide margins in federal elections stretching back nearly two decades to 1996.
ALEC has endured a wave of high-profile departures in the past. In 2012, a collection of major corporations, including Blue Cross/Blue Shield, Coca-Cola, PepsiCo, and Kraft left the organization after public outcry over the group's then-sponsorship of controversial "Stand Your Ground" laws, which came under heavy scrutiny during the Trayvon Martin case. The organization no longer works on legislation related to firearms.
In recent months, however, ALEC has again become a favorite target of left-leaning public-interest groups, due to its stance on global warming and its track record of backing legislation that opposes state renewable-energy mandates.
Environmental organizations, including the Sierra Club and the League of Conservation Voters, have accused ALEC of denying climate change, a charge the organization denies. Its model legislation for state lawmakers, however, runs counter to the scientific consensus on climate change, saying it remains unclear whether human emissions are changing global temperatures.
Watchdog organizations say Occidental's departure could indicate the start of a broader trend of companies not tied to Silicon Valley leaving the free-market organization due to public backlash against the company over its environmental policies.
"It says something that oil companies are leaving now," said Jay Riestenberg, a research analyst with Common Cause, a progressive organizing group that works to pressure companies to divest from ALEC. "They just don't see it as worth it anymore."