Interviews with party leaders in half a dozen Super Tuesday states suggest that the same vulnerabilities that plagued Mr. Biden beginning in Iowa — subpar organization, limited outreach to local Democrats and a late start to campaigning — are holding him back in the states that next week will dole out a third of the total delegates in the Democratic primary.

Mr. Biden’s on-the-ground operations, these Democrats said, are easily dwarfed by those of Mr. Sanders and Michael R. Bloomberg, the moderate former mayor of New York who has plainly cut into Mr. Biden’s standing in some of these states even as he faces his own mounting challenges in the race.

“Arkansas was, in my opinion, going to be a default Biden state,” said Michael John Gray, the chairman of the Democratic Party of Arkansas. “He hasn’t been here. Of all the campaigns, the least organized in Arkansas is Biden.”

Other candidates like Mr. Sanders, of Vermont, are already campaigning in Super Tuesday states, holding events that excite supporters and generate news media coverage. But Mr. Biden has planted himself elsewhere, after his fourth-place finish in Iowa and fifth-place New Hampshire result threw even his expected firewall of South Carolina into doubt.