Clinton says the project has been a distraction and she doesn't believe it's in the best interests of "what we need to do to combat climate change."
Hillary Clinton Presidential Candidate Profile | InsideGov
The former secretary of state had previously said she shouldn't take a position on the issue, saying she didn't want to interfere with the Obama administration's deliberations.
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But she has expressed her impatience in recent weeks over the pipeline decision, which is vigorously opposed by environmental activists.
Clinton announced her decision moments after Pope Francis arrived in Washington at the start of a closely watched visit to the United States.
