Vermont Sen. Bernie Sanders demanded an apology from Hillary Clinton Friday, after a video of an exchange between her and a Greenpeace activist went viral. Sanders is trailing Clinton in the race for the Democratic presidential nomination.

In the video, the activist accuses Clinton of accepting money from fossil fuel companies. She denies the claim and can be heard saying, "I am so sick of the Sanders campaign lying about that."

"We were not lying," Sanders said later. "We were telling the truth."

Both candidates campaigned in New York this week. The state holds its primary on April 19.

Brooklyn-native Sanders held a rally in the Bronx Thursday. Clinton, who is from Westchester, campaigned in upstate New York Friday. She also held a rally at Purchase College and in New York City earlier in the week.

Clinton made a stop at Syracuse University and attacked Sanders on the issue of free college tuition.

"My opponent talks about free college but you gotta read the fine print," Clinton said. "Anytime anyone tell you something is free, ask for the details."

New York is the second-largest delegate pool in the remaining primary states, behind California. Sanders still trails Clinton, but says a win in Wisconsin, and another win in New York, could change the course of the campaign.