Obama’s move comes as the White House continues to promote its environmental agenda and efforts to fight climate change. The Environmental Protection Agency this summer put forward new regulations limiting greenhouse gas emissions from power plants. And next month, Obama will attend the Paris climate talks run by the United Nations, he announced Friday. The White House is hoping to broker an international agreement committing every country to reduce carbon dioxide emissions and enact other policies to curb global warming.

The President has also stepped up his rhetoric on the need to address global warming, pushing back against Republicans and climate skeptics fighting his agenda.

“We know that human activity is changing the climate,” Obama said during a visit to Alaska in late summer. “We know that human ingenuity can do something about it. We’re even starting to see that we might actually have the political will to succeed. So the time to heed the critics and cynics is past. The time to plead ignorance is surely past. The deniers are increasingly alone, on their own shrinking island.”

In a statement Friday, Kerry said the climate impact was the key factor. “The critical factor in my determination was this: moving forward with this project would significantly undermine our ability to continue leading the world in combatting climate change,” he said.

Liberals and environmentalists, including top donors such as California’s Tom Steyer, who has committed tens of millions of dollars to fighting pro-pipeline political candidates, protested Keystone and made it a cause celebre among Democrats.

The project was a major issue during the 2012 presidential campaign, when GOP candidate Mitt Romney said he would approve the pipeline. Republican candidates in the 2016 race have also pledged to let the project go forward.

House Speaker Paul Ryan didn’t mince words in criticizing Obama’s action. “This decision isn’t surprising, but it is sickening,” Ryan said in a statement.

“So sad that Obama rejected Keystone Pipeline,” GOP presidential front-runner Donald Trump tweeted. “Thousands of jobs, good for the environment, no downside!”

In his speech, Obama said that he believed Keystone has had an “over-inflated role in our political discourse, and said the project’s potential to create jobs and the potential environmental threats were exaggerated.

“All of this obscured the fact that this pipeline would neither be the silver bullet to the U.S. economy proclaimed by some, or the death knell to climate proclaimed by others,” Obama said.

Obama also cited falling gasoline prices as another argument against the project.

“While our politics have been consumed by a debate about whether or not this pipeline would create jobs or lower gas prices, we’ve gone ahead and created jobs and lowered gas prices.”

The average price of regular gasoline hit $3.94 per gallon in April 2012 and stayed well above $3 for the rest of that election year. But this year, prices have been steadily below $3 per gallon.

Former Secretary of State Hillary Clinton has also been caught up in Keystone politics. In October 2010, Clinton indicated she was “inclined” to approve the project but has since backed away from that stance, and in September said she opposes it. Fellow Democratic presidential candidates Bernie Sanders and Martin O’Malley also oppose the pipeline, and Clinton faced criticism from the left for not taking a firm stance.

Sanders noted his long-standing opposition to the project in a statement Friday. “It is insane for anyone to be supporting the excavation and transportation of some of the dirtiest fuel on earth,” he said. “As someone who has led the opposition to the Keystone pipeline from Day 1, I strongly applaud the President’s decision to kill this project once and for all.”

Friday afternoon, Clinton tweeted her approval.

“The right call. Now it’s time to make America a clean energy superpower. -H,” she tweeted.