Watch Bernie double down on Clinton attack: Maybe Americans should wonder about your qualifications

Presidential candidate Bernie Sanders on Thursday morning again said that rival Hillary Clinton was not qualified to be president, vowing his campaign will fight back against recent attacks from the former Secretary of State.

The Vermont senator and Clinton have had a relatively polite rivalry in their competing bids for the Democratic nomination, but with the race tightening and Sanders winning in a string of state caucuses and primaries, the fight has gotten nastier in recent days.

"This campaign will fight back," Sanders said, according to ABC News. "When you have headlines in the Washington Post, Clinton questions whether Sanders is qualified to be president, my response is, 'well if you want to question my qualifications, let me suggest this. Maybe the American people might wonder about your qualifications, Madame Secretary."

He then criticized Clinton for voting for the Iraq war while a New York senator, and some trade agreements he says have hurt American workers, among other things.

Watch Sanders' comments, as posted by ABC, here:

WATCH: In press conference, @BernieSanders says Clinton questioned his qualifications first: "We will fight back" https://t.co/YiM4O3UoPn

— ABC News Politics (@ABCPolitics) April 7, 2016