On Wednesday, Clinton reiterated that sentiment.
"I thought this would be decided by now. Therefore I can tell you whether I agreed or I disagreed. But it hasn’t been decided and I feel now I got a responsibility to you and other votes who ask me about this," she said when asked by a young woman to give her position.
Clinton, who said she will be rolling out a North America climate change plan in the coming weeks, came out against the pipeline because, she said, "I don’t think it’s in the best interest of what we need to do to combat climate change."
"I do believe there is a lot of work to be done. We have a lot of pipelines, that are leaking, they need to be repaired, they are dangerous, they are leaking methane, they are at risk of causing damage. So I want to put thousands of Americans to work who are not only going to fix those old pipelines, but also we’ve got railcars transporting oil and I want those railcars, and the rail beds and the tracks they are on to be repaired," Clinton explained.
Both candidates released statements responding to Clinton's announcement.
“As a senator who has vigorously opposed the Keystone pipeline from the beginning, I am glad that Secretary Clinton finally has made a decision and I welcome her opposition to the pipeline," Sanders said in a statement. "Clearly it would be absurd to encourage the extraction and transportation of some of the dirtiest fossil fuel on the planet."
Former Maryland Governor O'Malley slammed Clinton for taking so long to take a position.
"On issue after issue--marriage equality, drivers licenses for undocumented immigrants, children fleeing violence in Central America, the Syrian refugee crisis, and now the Keystone Pipeline, Secretary Clinton has followed--not forged--public opinion. Leadership is about stating where you stand on critical issues, regardless of how they poll or focus group. " his statement read.
ABC News' MaryAlice Parks contributed to this report.