Bernie Sanders. Scott Olson/Getty Images Bernie Sanders and Hillary Clinton's presidential campaign exchanged some of their fiercest blows yet Wednesday and Thursday as the Democratic presidential primary heats up.

The focal point of the brouhaha was Sanders linking Clinton to the Panama Papers scandal and implying that she wasn't qualified for the presidency.

In response, Clinton's team tore into Sanders and said he had reached a "new low."

The Sanders campaign announced Wednesday night that it was responding in kind to Clinton's "attempt to portray him as unqualified for the White House." The Sanders press release linked to a Washington Post story titled "Clinton questions whether Sanders is qualified to be president."

Sanders then connected Clinton, the Democratic frontrunner, to a Panamanian free-trade agreement, dubbed by the Sanders camp as "a trade pact exploited by wealthy individuals and profitable corporations to avoid paying taxes." On Sunday, the massive Panama Papers document leak revealed how wealthy individuals used a Panamanian law firm to avoid paying taxes.

The Vermont senator came out swinging.

"I don't think you are qualified if you supported the Panama free-trade agreement, something I very strongly opposed, which has made it easier for wealthy people and corporations all over the world to avoid paying taxes owed to their countries," Sanders said in the Wednesday press release, which quoted him at a rally.

Sanders also criticized Clinton and her super PAC for taking money from Wall Street banks and special-interest groups, for voting in favor of going to war in Iraq, and for supporting other free-trade agreements.

Hillary Clinton. Brian Killian/Getty Images

Though Sanders insisted he was only responding to Clinton's questioning of his qualifications for the Oval Office, her campaign stressed that she never personally made that argument.

Instead, Clinton said Sanders "hadn't done his homework" on banking policy and questioned whether he could deliver on his campaign promises to better the US economy.

"Really what it goes to is for voters to ask themselves: Can he deliver what he is talking about? Can he really help people? Can he help our economy? Can he keep our country strong?" Clinton said Wednesday.

In a series of Wednesday-night tweet, Clinton press secretary Brian Fallon defended his candidate and criticized Sanders for calling her unqualified:

Hillary Clinton did not say Bernie Sanders was 'not qualified.' But he has now - absurdly - said it about her. This is a new low. — Brian Fallon (@brianefallon) April 7, 2016

Facing long odds, Sanders is inventing grievances & nonexistent paths to the nomination to rile his supporters & keep fundraising spigot on — Brian Fallon (@brianefallon) April 7, 2016

Bernie Sanders, take back your words about Hillary Clinton. — Brian Fallon (@brianefallon) April 7, 2016

At a news conference on Thursday morning, Sanders sought to downplay his comments about Clinton and said she was the one who threw the first punch. He said he wanted to talk about policy differences and not the qualifications debate.