Joe Biden has sparked concern from top Democrats and thousands of voters across the US following on from a spate of gaffes at major events and his disastrous debate against Donald Trump. The US President, 81, on Monday stood firm against suggestions he should drop his candidacy and appealed for an end to the row over how the party moves forward in a sharply worded letter to Democrats. Legislators and fundraisers including Hollywood actor George Clooney have since joined calls for Biden to step aside after he appeared flustered and stumbled over his words in the presidential debate against Trum…