Washington | Joe Biden faces two make-or-break tests this week in his fight to hang onto the presidential nomination as Democrats meet on continuing to support him, just hours before he hosts NATO heads reportedly worried about his fitness to lead at a time of rising global challenges.

A defiant US president went on the offensive on Monday (Tuesday AEST), sending a letter to wavering congressional Democrats, and called into national television to insist he would not quit the ticket despite mounting pressure to hand over the nomination before the November election.