Presumed Republican presidential nominee John McCain unveiled a detailed plan Monday to reduce greenhouse gas emissions and signaled that he was moving toward backing global warming legislation the Senate may take up next month.

In a speech in Portland, Ore., the Arizona senator called for placing a cap on emissions and setting up a market-based trading program for businesses. Under such a plan, companies with excessive emissions could obtain credits from those that reduce their emissions below the required level.

McCain’s plan is far more ambitious than anything President Bush has endorsed, but it is less stringent than proposals from his Democratic rivals, Sen. Barack Obama of Illinois and Hillary Rodham Clinton of New York.

“Like other environmental challenges — only more so — global warming presents a test of foresight, of political courage and of the unselfish concern that one generation owes to the next,” McCain said in prepared remarks.

McCain has long backed mandatory global warming legislation, in contrast to many of his Republican colleagues. His new climate change plan appears to be part of a strategy to attract independent voters concerned about the environment.

However, McCain has not yet signed on to global warming legislation (S 2191) sponsored by his allies Joseph I. Lieberman, I‑Conn., and John W. Warner, R-Va. Backers of that bill are scrambling to find the 60 votes necessary to overcome a filibuster when the measure comes to the floor, which could happen as early as June 2.

Like McCain’s plan, the bill would address global warming through a market-based cap-and-trade program. It also includes targets and timetables similar to those McCain is backing.

But McCain has withheld his support for the bill until it contains more incentives for nuclear power plants, which do not emit greenhouse gases. McCain argues that a revitalized nuclear industry should be a key component of a strategy for tackling climate change.

“It doesn’t take a leap in logic to conclude that if we want to arrest global warming, then nuclear energy is a powerful ally in that course,” McCain said.

Lieberman and Warner plan to circulate a draft manager’s amendment later this week that could include new financial or other incentives for nuclear plants, addressing McCain’s objections. Campaign spokesman Tucker Bounds said McCain “has strong feelings about the inclusion of nuclear energy language, and he is optimistic that negotiations will ensure that is successfully added.”

Passage of the bill during this Congress is still seen as a long shot, even if McCain persuades some fellow Republicans to support it. Many Democrats also think their chances for enacting strong legislation are better next year, no matter who the next president is.

Environmental groups and their Democratic allies are skeptical of the nuclear industry’s potential and might turn against the legislation if a manager’s amendment includes nuclear power incentives.

“With growing numbers of Democrats and Republicans now opposed to Lieberman-Warner, it’s hard to see how this bill even makes it to the Senate floor in the next few weeks,” said James M. Inhofe, R-Okla., the Senate’s leading critic of global warming legislation.

McCain proposed lowering greenhouse gas emissions 15 percent below 2005 levels by 2020, with a 66 percent reduction by 2050. These targets are fairly close to those in the Senate bill, which would reduce total domestic emissions 18 percent to 25 percent below 2005 levels by 2020, and by 62 percent to 66 percent by 2050, according to an analysis by the World Resources Institute and the Natural Resources Defense Council.

But environmentalists argue that much stricter limits are necessary to avoid dangerous effects of global warming. Obama and Clinton are backing an 80 percent reduction below 1990 levels by 2050.

The Democratic presidential candidates also want to sell emissions allowances for companies to trade in the carbon emissions market. McCain wants to provide some of the allowances to industry for free, which is consistent with the Senate legislation.

“His proposal would give billions in allowances to companies causing the problem,” said Daniel J. Weiss, senior fellow and director of climate strategy at the Center for American Progress Action Fund.

McCain also has a controversial proposal to allow the unlimited use of what are known as emissions offsets. For example, a power plant could avoid having to cut greenhouse gas emissions by purchasing an offset to credit a farmer for planting trees that capture carbon dioxide.

Critics say the wide use of offsets would allow industries to avoid real reductions from their own smokestacks and open the door to market manipulation. The Senate bill would allow limited offsets.

McCain said offsets would lower the costs of compliance and provide a wealth of incentives for farmers and ranchers. “Through the sale of offsets — and with strict standards to assure that reductions are real — our agricultural sector alone can provide as much as 40 percent of the overall reductions we will require in greenhouse gas emissions,” he said.