US President Joe Biden insisted Monday he would not quit the election race, as the White House denied he had Parkinson's disease following a disastrous debate performance.

The 81-year-old dared Democratic critics to either challenge him at next month's party convention in Chicago or back him against Republican rival Donald Trump in November's vote.

The president lashed out in both a letter to Congress and a rare call to a television program, at the start of a critical week including a NATO summit in Washington where he will face fresh scrutiny.

"I am firmly committed to staying in the race," Biden wrote in the letter.

"It is time to come together, move forward as a unified party and defeat Donald Trump," he said. "It's time for it to end."

The embattled president followed up by phoning into MSNBC's "Morning Joe" television program to say he was "getting so frustrated by the elites" in the party.

"Any of these guys that don't think I should run -- run against me. Announce for president, challenge me at the........

© International Business Times