“The idea of cutting each other down somehow hurting us in the general election, whichever campaign did that would get a lot of pushback, pitchforks and torches from the voters,” Ms. Gibson said. “There’s just seemingly no tolerance for it.”

Also in play are memories of 2016, when the bitter campaign between Hillary Clinton and Bernie Sanders left many supporters of Mr. Sanders alienated enough to stay home or vote for a third-party candidate.

In place of any negativity has been a focus on generally positive, biographical messaging. More than 25 percent of all ads in both Iowa and New Hampshire were either a general positive message or one about a candidate’s character, according to Advertising Analytics. In Iowa, the top issue was health care, addressed in about 9 percent of ads. In New Hampshire, it was the economy, also addressed in about 9 percent of ads.

There is also the tsunami of ads from Michael R. Bloomberg, who isn’t spending on ads in the first four early states but has dropped more than $165 million on television and digital ads in Super Tuesday states and beyond, according to Advertising Analytics. But Mr. Bloomberg has also not gone on the attack against other Democrats.

The apprehension toward negativity among the Democratic candidates has largely been evident in real time as well; when the debates have been marked by hostile attacks — Representative Tulsi Gabbard attacked Senator Kamala Harris’s record as prosecutor, Ms. Harris went after Joe Biden’s record on busing, to name a couple — other candidates are often quick to try to tamp down any bubbling anger.

“I did not come here to listen to this argument,” interjected Senator Amy Klobuchar at the sixth debate as a heated discussion over a fund-raiser at a wine cave boiled over. She added that the only way Democrats would win was “not by arguing with each other, but by finding what unites us in getting this done.”

The relative comity among the candidates on the airwaves comes after Republican presidential primaries in 2012 and 2016 that were overwhelmingly negative and seemed to signal a new era for presidential campaigns. With the injection of billions of dollars into such races after the Citizens United decision, leading to the proliferation of deep-pocketed super PACs, an explosion of negative advertising was becoming the norm. The so-called Eleventh Commandment popularized by President Ronald Reagan, which declared that “thou shalt not speak ill of any fellow Republican,” seemed destined for history.