Clinton said she was opposed to the construction of the pipeline during a campaign stop in Iowa, while cable news channels were otherwise fixated on Pope Francis' arrival in the Washington, D.C.
"I think it is imperative that we look at the Keystone XL pipeline as what I believe it is: A distraction from the important work we have to do to combat climate change, and, unfortunately from my perspective, one that interferes with our ability to move forward and deal with other issues," she said, as quoted by NBC News. "Therefore, I oppose it."
Clinton previously declined to state a position on the proposed project, citing her role in evaluating the construction of the pipeline during her tenure as secretary of state.
"This is President Obama's decision, and I am not going to second-guess him, because I was in a position to set this in motion, and I do not think that would be the right thing to do," she said at a July town hall in New Hampshire, as quoted by The Huffington Post.
"If it's undecided when I become president, I will answer your question," she added at the time.
The Democratic frontrunner acknowledged her delay in stating a position on the Keystone XL pipeline at Tuesday's event, telling the person who asked her about it: "I feel now I've got a responsibility to you and other voters who ask me about this."
This post has been updated.