DES MOINES, Iowa — Hillary Rodham Clinton said Tuesday she opposes construction of the Keystone XL pipeline, breaking her longstanding silence over a project criticized by environmentalists as a threat to the planet’s climate.

The Democratic presidential candidate said she decided to speak out after concluding the ongoing debate over whether the pipeline should be built had become a distraction to larger efforts to fight climate change.

That distraction, she said, is “unfortunately, from my perspective, one that interferes with our ability to move forward to deal with the other issues. Therefore I oppose it.”

Clinton’s announcement came as she has ceded ground in some polls to Vermont Sen. Bernie Sanders, who has long opposed the project. It also followed the appearance of protesters at some of her recent campaign events holding signs that read, “I’m Ready for Hillary to say no KXL.”

The former secretary of state had previously said she shouldn’t take a position on the issue, because she didn’t want to interfere with the Obama administration as it considers whether to allow construction of a pipeline.