The tax credits can still be extraordinarily effective, said Michael Greenstone, a University of Chicago economist and former Obama administration official who co-wrote a study of the plan’s tax incentives for electricity production that estimated the plan would reduce projected emissions by 13% to 22% compared with doing nothing and would generate benefits of more than three times their costs. “What we really want to do is unleash the ingenuity of the American economy for the cheapest ton of CO2 to abate wherever it can occur,” said Mr. Greenstone, who added that the plan benefits from recent improvements in clean-energy technology. “Five or 10 years ago, these numbers would not look so good.”