The linchpin of California’s climate change agenda, a program known as cap and trade, has become mired in legal, financial and political troubles that threaten to derail the state’s plans to curb greenhouse gas emissions.

The program has been a symbol of the state’s leadership in the fight against global warming and a key source of funding, most notably for the high-speed rail project connecting San Francisco and Los Angeles.

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“If we cease to have [cap and trade], that’s going to send a very loud message to the rest of the world and one we may not want to send,” said Michael Wara, a Stanford law professor.