WASHINGTON—Breaking her cautious silence, leading Democratic presidential candidate Hillary Clinton came out Tuesday against the Keystone XL oil pipeline proposal.

Clinton’s decision further worsens the prospects for the TransCanada Corp. project that has long been a top economic and diplomatic priority for Prime Minister Stephen Harper.

Caught in a delicate spot between Democratic environmentalists who oppose Keystone XL and Democratic unions that support it, Clinton had insisted she would not say anything about the pipeline until she was elected. But she faced damaging public pressure from both sides: from chief rival Bernie Sanders and from Republicans who cited her evasions as reflective of her personality.

Clinton did not quite endorse the environmentalists’ argument that the pipeline would contribute to climate change. Instead, speaking in response to a question at a town hall event in Iowa, she called it “a distraction” from other “important work we have to do to combat climate change.”

“Therefore, I oppose it,” she said. “And I oppose it because I don’t think it’s in the best interest of what we need to do to combat climate change.”

TransCanada spokesman Davis Sheremata said pipelines are a safer and more environmentally efficient way to move oil than transporting it by train. And he said Keystone XL would allow the U.S. to get oil from Canada rather than Iran and Venezuela, “where American values of freedom and democracy are not shared.”

Justin Trudeau’s Liberals support Keystone. Thomas Mulcair’s NDP is opposed.

The $8-billion (U.S.) pipeline would carry hundreds of thousands of barrels of oil per day from the Alberta oilsands and the shale fields of North Dakota and Montana down to Nebraska, where it would connect with existing pipelines running down to refineries in Texas.

The State Department, which Clinton led as secretary of state between 2009 and 2013, concluded in 2014 that Keystone XL was unlikely to have a significant impact on carbon emissions, since oil from the high-emissions oilsands would get to market with or without the new pipeline. But it cautioned that this might not be true if oil prices dropped. They have.

Every major Democratic candidate is now on record against Keystone XL. President Barack Obama, who has promised to make a decision before the end of his presidency, has offered only criticism of the project in 2015.

TransCanada’s hopes appear to lie with Republicans: every major Republican candidate supports Keystone XL. If Obama rejects the proposal, TransCanada could apply again when he leaves office.

“Hillary Clinton finally says what we already knew. She favors environmental extremists over U.S. jobs,” Republican candidate Jeb Bush wrote on Twitter.