LONDON: As chances rise of a Joe Biden-Donald Trump rematch in the US presidential election, America's allies are bracing for a bumpy ride.

Many worry that a second term for Trump would be an earthquake, but tremors already abound — and concerns are rising that the US could grow less dependable regardless of who wins. With a divided electorate and gridlock in Congress, the next American president could easily become consumed by manifold challenges at home — before even beginning to address flashpoints around the world, from Ukraine to the Middle East.