"I am firmly committed to staying in the race," Biden wrote in a letter to his fellow Democrats.

"It is time to come together, move forward as a unified party and defeat Donald Trump."

Biden said Monday he was confident voters still back him despite concerns over his age and urged any rivals to challenge him at the upcoming Democratic Party convention.

He told MSNBC's "Morning Joe" television program that he had held several rallies after a disastrous debate performance to "make sure I was right that the average voter out there still wanted Joe Biden, and I'm confident they do."