THE ENVIRONMENTAL Protection Agency will soon unveil its most anticipated new rules in a generation, the shape of which will help determine how well the United States responds to the threat of climate change. There will be fierce debate about them before they become final. Critics will try to undermine the effort by insisting that the rules be weak, poorly designed and unnecessarily expensive. Instead, the EPA should make them ambitious, fair and flexible.

Congress failed to pass a comprehensive climate change policy in 2010, leaving the EPA at the center of the federal anti-carbon effort. Congress decades ago empowered the agency to regulate a range of air pollutants under the Clean Air Act, so it can apply various extant authorities to cut U.S. greenhouse gas emissions without further congressional action. One set of rules, already released, limits the construction of power plants that would produce too much carbon dioxide, all but banning new, noxious coal-fired power stations. That’s good.

Now comes the big one: The EPA is poised to demand cuts from existing power plants. They are responsible for nearly half of the nation’s greenhouse emissions. The agency is reportedly ready to require carbon emission reductions in the power sector of about 25 percent by 2030. It is still determining whether the baseline from which the reduction will be measured will be carbon dioxide levels in 2005 or in a more recent year. The more recent the baseline, the tougher the standard will be to meet, since emissions have been declining. The EPA should use the toughest baseline it can justify based on the marginal cost of reducing carbon dioxide under its program.

That will depend on the outcome of another major dispute: How flexible will the rules be? The EPA wants to set broad emissions goals for states, then allow state governments and power companies flexibility in meeting them — by installing new technologies at power plants or investing in efficiency or renewable energy off site. Critics contend that the EPA can’t do that under the Clean Air Act. The extent of EPA authority will be decided in court. But there is no doubt about what the better policy would be. Instead of limiting the EPA to plant-by-plant mandates, it is far better to demand deeper emissions cuts but offer the power sector leeway to meet them inexpensively and creatively.

A node of opposition is coalescing around the perverse notion that if there are to be new rules, they must be irrationally painful. If that line of argument succeeds, required emissions reductions may have to be lower than planned.

What’s at stake is more than a few gigatons of carbon dioxide here or there. The world is closely watching what emerges from the EPA rulemaking process. Other nations will use the results to gauge this country’s seriousness — and the extent to which they can find moral cover to delay cuts they should be making. The United States should not offer them any.