But some policy makers haven’t been willing to acknowledge this. They continue to look for a solution without downsides. For them, a tempting option is a series of new rules requiring people to use cleaner energy. In a few cases, such rules really are a free lunch, in that they force people to take steps  like home insulation  that save money. But most rules increase costs. They force people away from the energy sources they are now using.

The classic example is the fuel economy rules from the 1970s that required car companies to make fewer gas guzzlers. The newly imposed scarcity of guzzlers, in turn, increased their price. But the relationship wasn’t obvious. Americans do not think of fuel economy rules as a tax on large vehicles.

This explains why the rule-based approach seems to be the best bet for winning Republican votes. Senator Richard Lugar, Republican of Indiana, has proposed new rules not just for vehicles but also for appliances, building codes and power plants.

If these regulations were tough enough, they could make a difference, as the fuel economy rules have. So some Democrats and environmentalists see this approach as their best remaining chance. “There’s a way Senator Reid and the president could manage this to get a very strong energy bill,” Hal Harvey, head of the ClimateWorks Foundation in San Francisco, said. “The victory is there for them to have.”

On the other hand, such rules would require government regulators to make all kinds of decisions  about which dishwashers qualified as efficient, about which alternative energies power plants had to use and the like. Businesses and consumers couldn’t look simply for the cheapest solution, as they could if Congress put a price on carbon. They would have to comply with specific provisions.

The result would almost certainly be higher, albeit better disguised, costs than with a carbon cap or tax. Even many advocates admit that new rules won’t do enough, on their own, to reduce emissions and slow warming. Only a cap or a tax can accomplish that at a reasonable cost.

Thus the opposition among other Democrats and environmentalists to accepting the Lugar approach as a compromise  and Mr. Reid’s difficulty in finding 60 votes for it.