Joe Biden dismisses demands that he should step aside for a more energetic presidential candidate ahead of the autumn elections in the USA.
"I am firmly determined to continue", writes the president in a letter to his party colleagues.
He urges them to rally behind him.
"It's time to come together, to move forward as a united party and defeat Donald Trump", writes Biden.
A growing divide over Biden's leadership has emerged within the Democrats after the president's criticized debate performance against Trump last week.
Several Democrats have openly demanded that the president step down, citing his inability to effectively counter Trump.
New critical voices
During a conversation on Sunday between leading Democrats in the House of Representatives, at least four new voices – including the prominent politician Jerry Nadler from New York – are said to have joined the demands, reports the AP news agency. Several others are said to have expressed their concern over Biden's ability to win the election, but without directly demanding that he step down.
Other top Democrats have simultaneously expressed their wholehearted support for the president.
And Biden himself does not budge an inch in his two-page letter.
"The question of how we should move forward has been discussed extensively for over a week. It's time for it to stop now. We have a task. To defeat Donald Trump", writes the president.
"Don't care"
In a follow-up telephone interview with MSNBC on Monday morning, Biden says he has support from "ordinary Democrats". He admits at the same time that influential Democrats have backed the demands for him to step down.
They are heavy names, but I don't care what the heavy names think, says Biden.
The president further claims to be confident that he will receive voters' support despite his age. In a jab at his critics, he adds that they are free, if they mean business, to challenge him at the Democrats' upcoming party convention where Biden is to be formally nominated.
