"I'VE ALWAYS SAID it's a serious long-term issue," said President Bush on Tuesday, when asked about his climate change policy. "And my administration isn't waiting around to deal with the issue. We're acting." Clearly, the president's definition of "acting" is different from ours. For the past five years, Mr. Bush has responded to all talk of climate change with demands for more scientific evidence. Now, a former employee of the government office that coordinates climate change research has released documents showing that a White House aide has been doctoring official reports on climate change precisely to avoid "acting" on the scientific evidence. The aide, Philip A. Cooney -- chief of staff for the White House Council on Environmental Quality -- is alleged to have made dozens of changes to official reports, inserting qualifiers designed to cast doubt on findings about climate change and to play down the link between climate change and industrial greenhouse gas emissions.

The result, though, is that the White House may soon be the last institution in Washington that doesn't believe that the threat of climate change requires something more than new adjectives. This week, when the Senate is scheduled to begin debate on its new energy bill, more than one piece of climate change legislation may well be proposed as an amendment. For the first time, one of them may well pass. In the past, Sens. John McCain (R-Ariz.) and Joseph I. Lieberman (D-Conn.) have proposed, but never got enough votes for, legislation establishing a mandatory cap-and-trade system that would allow companies to buy and sell the right to emit greenhouse gases, so that those who can cut their emissions the most cheaply do so first. Now, however, Sen. Jeff Bingaman (N.M.), the ranking Democrat on the Energy Committee, has prepared -- in consultation with Republican colleagues -- an alternative amendment, one that would set up a cap-and-trade system, somewhat less rigorously, but far more cheaply, than the McCain-Lieberman bill. It is beginning to attract a surprisingly broad level of Democratic and Republican support.

This new legislation is based on a proposal put together by the National Commission on Energy Policy, a bipartisan group that includes industry chief executives, environmentalists and scientists. According to a recent Energy Department analysis, that group's cap-and-trade system would cause only a minimal rise in electricity prices, and would not, unlike the McCain-Lieberman bill, lead to a sharp reduction in the use of coal. The legislation would also allow Congress to continually reassess the national cap on greenhouse gases, depending on what measures are being taken in other countries. Those two measures go a long way to answer those critics who claim addressing this issue in any way will render the U.S. economy uncompetitive.

Even if this amendment never becomes law, its passage would deal a significant psychological blow to the last holdouts in the White House -- which is why Republican senators should resist the inevitable White House pressure not to vote for it. After all, they belong to a party that often claims to believe in the superiority of market mechanisms over regulation. Now that they have an opportunity to act by letting markets determine how to manage emission reductions, they should leap to take it.