Both Bernie Sanders and Martin O'Malley criticized Hillary Clinton for being late to oppose the Keystone XL Pipeline, which President Obama rejected Friday.

"For me as opposed to maybe some other unnamed candidates, the issue of Keystone was kind of a no brainer, it never made sense to me from day one," Sanders said during the South Carolina Democratic presidential forum.

The democratic socialist from Vermont was one of the first senators to oppose the pipeline. O'Malley, the former governor of Maryland, has also been a persistent pipeline critic.

Clinton was the last Democratic candidate to reveal her stance on the pipeline, as she previously said that she would wait for the Obama administration come to a decision first. As the media and voters alike pressed Clinton for answers, she finally stated that she was against the construction of the pipeline in September.

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