More than anything, the recent Sanders broadsides reflect a political strategy he has carried out in previous campaigns: the use of blunt criticisms, sarcastic asides and a thundering style against his opponents.

In the 1986 race, Mr. Sanders argued that he would be a strong feminist and do more for women than Ms. Kunin had. While granting that Ms. Kunin was “not corrupt,” he questioned if she had the same “courage” that he had. He repeatedly challenged her credentials as a fellow progressive, using some of the same language he aims at Mrs. Clinton. In the end, he damaged Ms. Kunin politically, as some Clinton supporters and political analysts think he may do in the current race.

“In a tough fight, Bernie is hardly the all-positive, all-substance guy that he claims to be,” said Garrison Nelson, a longtime political science professor at the University of Vermont.

Ms. Kunin was not the only foe that Mr. Sanders attacked with insinuations, as opposed to the more overtly negative television ads that Mr. Sanders has forsworn. In his 1990 race for Congress, he frequently laid political bait for his Republican opponent and relished watching him stumble. And in his 2006 Senate campaign, Mr. Sanders relentlessly linked his moderate Republican opponent with George W. Bush and Dick Cheney, and accused him of running “the most negative, dishonest campaign in the history of the state of Vermont.”

While such tactics are not unusual in many campaigns, Mr. Sanders has long tried to claim the high road. Yet if his past opponents remember anything about him, it’s Bernie the brawler.

“The way he kept tagging me as a typical rich guy who only cared about rich Republicans — it was very tough, and very effective,” said Richard Tarrant, a software executive who was the Republican Senate nominee in 2006 and ran many aggressive television ads. “Bernie knew that I earned my money myself, that my wealth was first-generation. But that didn’t matter.”