Former US President Barack Obama on Wednesday assailed his successor, Donald Trump, as deeply unfit for the office he occupies and argued that voting for his former No. 2, Joe Biden, was necessary to ensure the survival of American democracy.

Obama blamed Trump for the 170,000 Americans who have died from the coronavirus, the millions of jobs lost to the ensuing recession and the diminishment of the country's democratic principles at home and abroad.

On Twitter, Trump responded to Obama's appearance in all capital letters, suggesting Obama's decision to endorse Biden only after his Democratic rivals dropped out indicated doubts about Biden's candidacy.