Who could replace Biden as Democratic nominee?

A number of Democratic members of Congress are calling on President Joe Biden to end his re-election campaign.

A disastrous debate against Republican challenger Donald Trump has fanned flames of discontent among lawmakers in the House.

And a series of polls taken since then suggest Mr Trump is tightening his grip on the race as concerns about Mr Biden's age appear to grow among voters.

This has raised questions about who could replace him on the presidential ticket, should Mr Biden change his mind about staying in the race.

Here...