In Iowa, Biden confronts a growing threat: Pete Buttigieg.

DES MOINES, IA - NOVEMBER 01: Democratic presidential candidate, former Vice President Joe Biden speaks during the Iowa Democratic Party Liberty & Justice Celebration on November 1, 2019 in Des Moines, Iowa. Fourteen presidential are expected to speak at the event addressing over 12,000 people. (Photo by Joshua Lott/Getty Images)

Joe Biden dropped to fourth place in Iowa, according to a new poll released Friday, his worst showing to date in the pivotal early state.

A few hours later, at the largest gathering to date for any 2020 event, it was clear why…