SAN FRANCISCO — Once again, a bill to tax emissions of greenhouse gases has been introduced in Congress, this time with Republican sponsorship. Just about every economist thinks putting a price on these emissions would reduce them and hasten the shift toward clean energy, slowing climate change. This latest push is, in principle, a good idea.

It is also a political fantasy .

Attempts to tax or otherwise price emissions have been made in Congress for a quarter-century now. The first ambitious bill, in 1993, got whittled down to a modest increase in the gasoline tax, and the rest of these efforts have failed miserably, even when Democrats had strong congressional majorities. By the time this latest proposal was even introduced, the House of Representatives had already passed a resolution declaring any emissions tax dead on arrival.

Every indication is that these emissions-pricing plans will keep failing, even if Democrats manage to seize the House in November. But a different approach might stand a better chance in Congress — one that would focus on building more clean energy, rather than taxing emissions. This could be accomplished by setting a national clean-energy standard.

This policy would require the share of American electricity from low-emitting sources to increase steadily over time. States would be given flexibility in how they meet the national goal — that is, which energy sources to embrace — but they would have to meet it, most likely by fining utilities that failed to comply.