Five possible outcomes of the 2020 US presidential election

Voters cast their ballots during early voting in Cambridge, Massachusetts for the 2020 presidential election (Elise Amendola/AP)

Democratic candidate Joe Biden heads into the 2020 US presidential election as favourite to beat Republican President Donald Trump.

But while Biden has enjoyed a comfortable lead in nationwide opinion polls for several months, his lead in some of the key battleground states is much smaller.