WASHINGTON — When Barack Obama first ran for president, being green was so popular that oil companies like Chevron were boasting about their commitment to renewable energy, and his Republican opponent, John McCain, supported action on global warming.

As Mr. Obama seeks re-election, that world is a distant memory. Some of the mightiest players in the oil, gas and coal industries are financing an aggressive effort to defeat him, or at least press him to adopt policies that are friendlier to fossil fuels. And the president’s former allies in promoting wind and solar power and caps on greenhouse gases? They are disenchanted and sitting on their wallets.

This year’s campaign on behalf of fossil fuels includes a surge in political contributions to Mitt Romney, attack ads questioning Mr. Obama’s clean-energy agenda, and television spots that are not overtly partisan but criticize administration actions like new air pollution rules and the delay of the Keystone XL oil pipeline from Canada.