It may have been a late night comedy show, but Bernie Sanders was all business defending his criticism of Hillary Clinton on "Late Night with Seth Meyers" Thursday.

The Vermont senator stopped by the NBC show while campaigning in New York for the upcoming April 19 primary in New York state and was pressed by host Seth Meyers on the escalating tension between Sanders and the former secretary of state. 

"After we won in Wisconsin, which was our sixth victory in seven caucuses and primaries, I think the Clinton campaign has been getting a little bit nervous, and I think they have been getting more negative," Sanders said. "I hope very much that we can have an issue-oriented campaign, but if people attack me and distort my record, we will respond."

Meyers specifically asked Sanders about controversial comments Sanders made calling Clinton "unqualified" to be president. Sanders' response: She started it. 

"It was said after [Clinton] and her campaign said that I was unqualified," Sanders insisted. 

Despite Sanders' accusations, Clinton stopped just short of calling Sanders unqualified in an interview on MSNBC's "Morning Joe" Wednesday — the interview appears to be the source of Sanders' scorn. 

"I think [Sanders] hadn’t done his homework, and he’d been talking for more than a year about doing things that he obviously hadn’t really studied or understood," Clinton said. "That does raise a lot of questions."

Reading between the lines, Sanders decided to fire back at a campaign rally in Philadelphia that night. 

"She has been saying lately that she thinks that I am ‘not qualified’ to be president," Sanders said. "Well, let me just say in response to Secretary Clinton: I don’t believe that she is qualified, if she is, through her super PAC, taking tens of millions of dollars in special interest funds. I don’t think that you are qualified if you get $15 million from Wall Street through your super PAC."

The tense back-and-forth comes as the New York primary looms over both candidates. There are 247 delegates at stake in the crucial contest.