Rufus Gifford was ambassador to Denmark during President Obama's second term and later ran for Congress.
Biden's former rival for the Democratic presidential nomination lauds his empathy and integrity.
The Democrats must defeat "the most dangerous president in the modern history of this country," Sanders says.
The move clears the path for Joe Biden to win the nomination and face Trump in November.
It's a make-or-break night for Sanders as Biden tries to build on his momentum from Super Tuesday.
Klobuchar's exit comes a day after Pete Buttigieg's and represents a consolidation of moderates in the Democratic race.
The results in a state seen as a must-win for Biden may buoy his campaign.