WASHINGTON — Calling it a moral obligation, President Barack Obama unveiled the final version of his plan to dramatically cut emissions from U.S. power plants, as he warned anew that climate change will threaten future generations if left unchecked.

Touting the plan at a White House event on Monday, Obama said the unprecedented carbon dioxide limits are the “the single most important step” America has ever taken to fight climate change. He warned that because the problem is so large, if the world doesn’t get it right quickly, it may become impossible to reverse, leaving populations unable to adapt.

Opponents immediately announced they would sue the government to stop the rules from taking effect. West Virginia Attorney General Patrick Morrisey, speaking at a summit of Republican state attorneys general, said West Virginia would be among a group of states “launching an aggressive legal campaign.”

Power plants account for roughly one-third of all U.S. emissions of the heat-trapping gases blamed for global warming, making them the largest single source. Already Obama has curbed greenhouse gas emissions from other major sources, including vehicles.

The final version of Obama’s plan imposes stricter carbon dioxide limits on states than was previously expected: a 32 percent cut by 2030, compared with 2005 levels, the White House said. Obama’s proposed version last year called only for a 30 percent cut.

It also gives states an additional two years — until 2022 — to comply, yielding to complaints that the original deadline was too soon. States will also have an additional year to submit their implementation plans to Washington.

Obama was joined in the East Room by Environmental Protection Agency Administrator Gina McCarthy and by parents of asthma patients. The Obama administration has sought to draw a connection between climate change and increased respiratory illness in vulnerable populations.

As they prepared to sue the government, states and energy companies asked the EPA to put the rules on hold while legal challenges play out — a notion that White House press secretary Josh Earnest dismissed.