Hours after Bernie Sanders did an about-face, saying he believes Hillary Clinton's use of a private email server as secretary of state should be investigated, the Vermont senator further upped his rhetoric by claiming that he and the Democratic front-runner disagree on "virtually everything."

Sanders is trying to draw clearer distinctions between his liberal record and what he views as Clinton's flip-flops on major issues, including gay rights, the environment and trade, the Boston Globe noted. Fortunately, the former New York senator's decades-long career in government provided ample material for him to point out, Clinton's challenger told the newspaper.

"What is important is to look at is the record, the track record that Hillary Clinton has had for her long and distinguished career as a public figure," Sanders said.

The former Burlington, Vermont mayor's newfound aggressiveness comes as his presidential campaign seems to have hit a critical -- and difficult -- moment, according to Bloomberg. Despite the large crowds Sanders has managed to attract at events in key early-voting states, his poll numbers appear to have reached a peak, the business publication noted.

"Sanders is not so much declining, but has maximized his potential support and bumping up against his ceiling," said Ken Goldstein, a professor of politics at the University of San Francisco who analyzes polls for Bloomberg.

The socialist senator is increasingly trying to portray himself as an "honest" alternative to Clinton, whose image has been damaged by recent inquiries into her email habits, as well as into her handling of the Sept. 11, 2012, terrorist attack on the U.S. outpost in Benghazi, Libya.

"You asked me about the differences between Hillary Clinton and myself?" he told the Boston Globe. "I have taken tough votes throughout my entire life. You are looking at a guy who cast difficult votes."

Clinton spokesman Josh Schwerin, meanwhile, urged Sanders to return to a "campaign about issues," The Wall Street Journal noted.

"It's disappointing [that] Sen. Sanders and his campaign strategists have chosen to change direction and engage in the type of personal attacks that they previously said he wouldn't do," Schwerin said.