New York (CNN) Bernie Sanders may not want to go after Hillary Clinton directly, but his presidential campaign has shown over the last two days that it is are more than happy to punch back against the world of pro-Clinton super PACs.

The tweet linked to an MSNBC story on how Brock, the head of a number of pro-Clinton PACs and longtime Clinton operative, wrote in his new book that Move On's efforts to draft liberal Sen. Elizabeth Warren to run for president were "cynical" and "misguided."

In "Killing the Messenger," Brock writes that Move On "rather cynically see the opportunity to build their e-mail lists and make some money by backing 'a more progressive alternative' against Hillary Clinton."

For much of 2014, MoveOn and a coalition of other liberal organization were trying to draft Warren to run for president. While the movement failed to get Warren in the race -- she has emphatically said a number of times she won't run -- it provided fodder for Republicans to call out Clinton for not being liberal enough for the Democratic Party.

This is not the first time Sanders has hit back against pro-Clinton super PAC attacks.

Earlier this week, Correct the Record -- a pro-Clinton super PAC that works on messaging and research -- sent an email to the Huffington Post that linked Sanders to Hugo Chavez, the late leader of Venezuela, and Jeremy Corbyn, the new United Kingdom Labour leader. The email was intended to be off-the-record, but the publication published a story on Monday, reporting that they received the email "without any agreement that it would be off the record."

In response, Sanders campaign sent two fundraising emails about the negative attacks against them.

"Yesterday, one of Hillary Clinton's most prominent Super PACs attacked our campaign pretty viciously," read the fundraising email in Sanders' name. "They suggested I'd be friendly with Middle East terrorist organizations, and even tried to link me to a dead communist dictator."

He added, "They'll keep trying ... unless we make them pay a price for their attacks. ... They should not underestimate us."

Later in the day, Jeff Weaver, Sanders' campaign manager, sent a similar fundraising email that said "this will not be the last time a Super PAC considers attacking our campaign."

The back and forth between pro-Clinton super PACs and Sanders campaign appears very different from the public relationship between Clinton and Sanders themselves.

Sanders has been hesitant to go after Clinton, stating instead that they have differences in policy opinions but that he likes and respects the former secretary of state.

Sanders has been surging in the polls lately and has caught Clinton in both Iowa and New Hampshire. Asked on Monday if she was worried about the Vermont senator, Clinton said, "I am not."

"He is doing a great job," Clinton said of Sanders.