Bernie narrowly leads the new Des Moines Register Iowa poll, with the top four still bunched up. The poll captures real sources of strength for him:

Sanders’ supporters are more likely than those who support the other leading candidates to say their minds are made up (59%), and they are “extremely” enthusiastic about their candidate (49%). Just 32% of Warren’s supporters describe themselves as extremely enthusiastic, and 26% each for Biden and Buttigieg.

“There was a thought that his support was a holdover from when he ran before and that that would evaporate,” Selzer said. “It certainly has not evaporated.”

She said Sanders is holding on to many of those who caucused for him in 2016 while also growing support among young and first-time caucusgoers. Those are notoriously difficult groups to turn out on caucus night, she said, but Sanders has done it before and appears to be on track to do so again.

He has retained support from 44% of those who say they caucused for him in 2016. Warren earns 20% of his former supporters.

And he leads the field with those younger than 35, earning 36% of their support; Warren follows him at 20%.