Updated: 7:16 p.m.

The U.S. Chamber of Commerce, which recently suffered high-profile defections because of its opposition to climate change legislation, is now facing a demand that the nation’s top doctors association resign from the business group over the chamber’s stand against health care overhaul efforts.

MoveOn.org has launched an effort to pressure the American Medical Association to withdraw from the chamber because of its lobbying against the Democratic legislation that recently was approved by the House.

“While the AMA has been publicly backing reform, they’ve also been helping fund a multimillion-dollar campaign to kill the bill through the membership dues they pay to the U.S. Chamber of Commerce,— MoveOn wrote in an e-mail.

A chamber official forcefully dismissed MoveOn’s tactics.

“This is part of an orchestrated attack designed to intimidate and harass the chamber and its members,— said the chamber’s Tita Freeman, adding that the AMA is part of the chamber’s Committee of 100, the most active 100 trade associations in the group.

“The AMA and chamber both acknowledge that change in our current health care system is needed but have different views on how best to achieve it,— AMA’s Rebecca Patchin said. “These differences, however, do not negate the common ground we do share.— The AMA backed the $1.2 trillion health care plan approved by the House, which includes a public insurance option.

At a meeting in Houston earlier this month, the leaders of the medical group beat back efforts by some members who had sponsored a resolution calling on the AMA to oppose any new public insurance option.

The chamber, meanwhile, has aggressively opposed the House health care plan, arguing it would not control costs and would jeopardize employer-based health insurance. The chamber has also run television ads to pressure moderate Democrats not to vote for the plan.

This is just the latest instance in which chamber members have been pressured to leave the organization because of its stance on issues. Earlier this year, a number of companies, including Pacific Gas and Electric Co., Apple Inc. and Exelon Corp., quit the chamber because of the its opposition to climate change legislation. Nike Inc. resigned from the chamber’s board of directors over climate change, too.

MoveOn launched an online campaign to pressure Toyota Motor Corp., which manufactures the Prius, to withdraw from the chamber because of its stand on climate change. However, the automobile company refused.

“We’ve been applying pressure to a number of companies that have positions that conflict with Chamber of Commerce positions,— said Adam Ruben, political director of MoveOn.