WASHINGTON — The new pollution rule the Obama administration announces Monday will be a cornerstone of President Barack Obama’s environmental legacy and arguably the most significant U.S. environmental regulation in decades.

But it’s not one the White House wanted.

As with other issues, the regulation to limit the pollution blamed for global warming from power plants is a compromise for Obama, who again finds himself caught between his aspirations and what is politically and legally possible.

It will provoke a messy and drawn-out fight with states and companies that produce electricity, and might not be settled until the eve of the next presidential election in 2016, or beyond.

“It’s going to be like eating spaghetti with a spoon. It can be done, but it’s going to be messy and slow,” said Michael Gerrard, director of the Center for Climate Change Law at Columbia University.

At the crux of the problem is Obama’s use of a 1970 law that was not intended to regulate the gases blamed for global warming. Obama was forced to rely on the Clean Air Act after he tried and failed to get Congress to pass a new law during his first term. When the Republicans took over the House, the goal became impossible.

The new rule, as the president described it in a news conference in 2010, is another way of “skinning the cat” on climate change.

“For anybody who cares about this issue, this is it,” Heather Zichal, Obama’s former energy and climate adviser, said in an interview with The Associated Press. “This is all the president has in his toolbox.”

The rule will tap executive powers to tackle the single largest source of the pollution blamed for heating the planet: carbon dioxide emitted from power plants. They produce about 40 percent of the electricity in the nation and about one-third of the carbon pollution that makes the U.S. the second largest emitter of greenhouse gases.

“There are no national limits to the amount of carbon pollution that existing plants can pump into the air we breathe. None,” Obama said Saturday in his weekly radio and Internet address.

“We limit the amount of toxic chemicals like mercury, sulfur, and arsenic that power plants put in our air and water. But they can dump unlimited amounts of carbon pollution into the air. It’s not smart, it’s not safe, and it doesn’t make sense,” he said.

While Obama has made major reductions in carbon pollution from cars and trucks by increasing fuel efficiency, manufacturers cooperated after an $85 billion government bailout.

His rule requiring new power plants to capture some of their carbon dioxide and bury it underground, while significant, has little real-world effect because few new coal plants are expected to be built due to market conditions.

Both those rules also prescribed technological fixes or equipment to be placed on the automobile or power plant.

The rule released Monday, though, would allow states to require power plants to make changes such as switching from coal to natural gas or enact other programs to reduce demand for electricity and produce more energy from renewable sources.

They also can set up pollution-trading markets as 10 other states already have done to offer more flexibility in how plants cut emissions. Plans from states won’t be due until 2016, but the rule will become final a year before.

That hasn’t stopped the hoopla over the proposal.

Some Democrats worried about re-elections have asked the White House, along with Republicans, to double the length of the rule-making comment period, until after this November’s elections.

The Chamber of Commerce said the rule would cost $50 billion to the economy and kill jobs. Harvard University said the regulation wouldn’t just reduce carbon but also would have a beneficial side effect: cleansing the air of other pollutants.

Environmental groups, meanwhile, are taking credit for helping shape it and arguing it would create jobs, not eliminate them.

Rep. Nick Rahall, a Democrat from West Virginia, which gets 96 percent of its power from coal, said Thursday that while he didn’t have the details, “from everything we know we can be sure of this: It will be bad for jobs.” Rahall faces a difficult re-election in November.

Obama said such pessimistic views are wrong.

“Now, special interests and their allies in Congress will claim that these guidelines will kill jobs and crush the economy,” Obama said in his address. “Let’s face it, that’s what they always say.”

Environmental Protection Agency Administrator Gina McCarthy and other government officials have promoted the proposal’s flexibility as way to both cut emissions and ensure affordable electricity. But that flexibility could backfire.

Some states, particularly those heavily reliant on fossil fuels, could resist taking action, leading the federal government to take over the program. That happened in Texas when it initially refused to issue greenhouse gas permits through another air pollution program.

Lawyers for states and industry also are likely to argue that controls far afield of the power plant violate the law’s intent.

The rule probably would push utilities to rely more on natural gas because coal emits about twice as much carbon dioxide. The recent oil and gas drilling boom in the U.S. has helped lower natural gas prices and, by extension, electricity prices. But it still generally is cheaper to generate power with coal than with natural gas. Also, natural gas prices are volatile and can lead to fluctuations in power prices.

The rule will push the U.S. closer to the 17 percent reduction by 2020 it promised other countries at the start of Obama’s presidency, it will fall far short of the global reductions scientists say are needed to stabilize the planet’s temperature. That’s because U.S. fossil-fueled power plants account for 6 percent of global carbon dioxide emissions.