Bernie Sanders
Bernie SandersReuters

Following a string of big primary victories, Vermont Senator Bernie Sanders launched his most direct attacks on former Secretary of State and Democratic frontrunner Hillary Clinton.

Speaking to a crowd in Pennsylvania on Wednesday, Sanders told supporters that his opponent for the Democratic nomination was “unqualified”, noting Clinton’s support for the Iraq war and close connections to Wall Street.

"Well let me, let me just say in response to Secretary Clinton: I don't believe that she is qualified if she is, if she is, through her super PAC, taking tens of millions of dollars in special interest funds," said Sanders. "I don't think you are qualified if you get $15 million from Wall Street through your super PAC."

“I don't think you are qualified if you have voted for the disastrous war in Iraq. I don't think you are qualified if you've supported virtually every disastrous trade agreement, which has cost us millions of decent paying jobs. I don't think you are qualified if you supported the Panama free trade agreement, something I very strongly opposed and which, as all of you know, has allowed corporations and wealthy people all over the world to avoid paying their taxes to their countries."

Sanders claimed that Clinton had used similar language in describing him, an assertion Clinton spokesman Brian Fallon said was untrue.

"Now the other day, I think, Secretary Clinton appeared to be getting a little bit nervous. We have won, we have won seven out of eight of the recent primaries and caucuses. And she has been saying lately that she thinks that I am, quote unquote, not qualified to be president."

Sanders has won the last six contests against Clinton, yet observers largely dismiss the possibility of the self-described democratic socialist winning the nomination, in large part due to Clinton’s enormous lead with superdelegates.

Party leaders are given a free vote in the Democratic National Convention, and are often referred to as superdelegates. They are unbound and not selected by any primary vote or state caucus. More than 90% of superdelegates who have pledged support thus far have gone to the Clinton camp, leaving Sanders with a steep climb to the nomination.