"There was a change in tone on the part of the Clinton people," he said. "They said we're not going to be nice to Bernie Sanders anymore, we're getting beaten every week. We're going to start beating him up when we go to New York City."
Coming off of his win in Wyoming on Saturday, Sanders said he has the momentum to win the nomination.
"There's no question I think the momentum is with us," he said. "We have come a really, really long way and I think we can win this."
While Sanders has won seven contests in the last eight states -- Idaho, Utah, Alaska, Hawaii, Washington, Wisconsin and Wyoming -- he still trails Clinton by more than 200 pledged delegates. But he sounded confident he could continue cutting her lead.
"In the last three and a half weeks, we have reduced her margin by a third," Sanders said
Next, the race heads to New York, where both candidates have strong ties: Sanders was born and raised in Brooklyn and Clinton has been a New York resident since 2000.
Sanders believes he could pull off a win in the Empire State.
"We are working really hard here. We want to win. We think we've got a shot to win and if we win here, it will be a major boost in Pennsylvania, to other states and out west," he said.
Sanders also declined to apologize for comments he made last week, saying he didn't believe families of gun violence victims should be able to sue gun manufacturers while referring to pending lawsuit filed by relatives of Sandy Hook mass shooting victims.
Sanders had voted for the Protection of Lawful Commerce in Arms Act in 2005, which shields firearms manufacturers and dealers from liability lawsuits, though he has since co-sponsored a bill that could repeal the measure.
"I get a little bit upset when one of the great, horrific tragedies in modern American history -- unspeakable -- becomes a political issue," Sanders said Sunday.