Three large companies have left the U.S. Climate Action Partnership, a coalition of more than two dozen companies and environmental groups lobbying Congress to pass a bill combatting climate change.

Oil companies ConocoPhillips and BP America, as well as machine giant Caterpillar Inc. said Tuesday they were not renewing their memberships in the group.

The defections were widely seen as a blow to congressional efforts to cap U.S. emissions of pollution-causing gases blamed for global warming. ConocoPhillips Chairman Jim Mulva said Congress has unfairly penalized domestic oil refineries and ignored natural gas companies.

The climate coalition includes some of the country's biggest electric utilities and oil companies, as well as five environmental groups.