Democratic presidential candidate Sen. Bernie Sanders (I-VT) said on Tuesday that it’s “hard to understand” why a candidate would remain silent on the Keystone XL Pipeline, reported This is an obvious jab at his opponent, Hillary Clinton.

Sanders is a staunch opponent of the Keystone XL Pipeline and has been one of the project’s most outspoken critics. “We must make significant reductions in carbon emissions and break our dependency on fossil fuels,” said Sanders. “That is why I have helped lead the fight in the Senate against the Keystone pipeline. It is hard for me to understand how one can be concerned about climate change but not vigorously oppose the Keystone pipeline.”

Clinton recently announced at a rally in Iowa her plan to establish a larger presence of sustainable energy should she become president. Adrienne Watson of the pro-Clinton group, Correct the Record, defended Clinton and said her “vision to continue to combat climate change has been applauded by the most ardent environmentalists.” However, Sanders doesn’t seem to completely buy Clinton’s environmental commitment.

The Keystone pipeline has been a major environmental talking point among Democrats for years. It has become a linchpin of sorts to gauge which politicians are really committed to the environmental agenda. That linchpin now has become part of the presidential discussion.