ADVERTISEMENT Biden skipped the confab to attend a Human Rights Campaign gala dinner in Ohio. Acting California Democratic Party Chairwoman Alexandra Gallardo-Rooker said Biden had promised he would visit California often in the near future. In his absence, Biden — and the idea that he is a safe choice for the nomination — became a punching bag.

"In my view, we will not defeat Donald Trump unless we bring excitement and energy into the campaign and unless we give millions of working people and young people a reason to vote and a reason to believe that politics is relevant to their lives. We cannot go back to the old ways. We have got to go forward with a new and progressive agenda," Sanders told the crowd.

"Some Democrats in Washington believe the only changes we can get are tweaks and nudges. If they dream at all, they dream small," she told delegates. "The time for small ideas is over."

Other candidates promised a break with the past, another clear allusion to Biden's pledge to return to a more civil style of the politics.

Biden, though, was not the only target as candidates polling low in the field sought to distinguish themselves.

Hickenlooper used his speech to attack the Sanders wing of the party, if not Sanders himself. A day before Sanders took the stage, delegates booed Hickenlooper as he urged his party to draw a clear distinction between socialism and Democratic candidates.

"If we don't draw a clear distinction between Democrats and our candidates and socialism, the Republicans will paint us into a corner that we can't get out of," Hickenlooper said in an interview. "In places like Ohio and Michigan and North Carolina and Wisconsin, places we have to win to beat Trump, we'll be starting out 10 yards behind."

The early contrasts are a hint of a sharper tone likely to define the Democratic contest during the summer and fall months. The change is borne in part of necessity — polls show Biden leading Sanders by a wide margin, with a handful of others trailing behind and a dozen or more candidates struggling even to register.

But party strategists, some affiliated with presidential campaigns and others unaligned, say there is an inherent danger in going negative in a primary before challenging Trump.

The unusually crowded field and the way the first debates will operate could cause some uncertainty for the leading candidates. It is not clear in what order the 20 candidates who make the debate stage, in two groups of 10, will appear. An attack line aimed at Biden may not work if the candidate preparing it doesn't appear alongside the former vice president.

"My story's really different from the other candidates running. I got my start in a 2-to-1 Republican district, won it twice, the second time by a 24-point margin," Gillibrand said.

Harris, the hometown senator who got her start in politics as San Francisco's district attorney, did not mention or allude to the rest of the Democratic field in her address to party faithful. Her biggest applause line came when she reiterated a call she had made earlier.

"We need to begin impeachment proceedings, and we need a new commander in chief," Harris said, to a standing ovation.