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. But the legality of cap and trade is being challenged in court by a business group, and questions are growing about whether state law allows it to operate past 2020.