Joe Biden is currently enjoying a 9-point lead in the US polls, but with less than a month to go the outcome is anything but certain.

While Biden can look with confidence to the polls, the US electoral system means that certain states may play a major role in the outcome. 

This year's swing states are worth 135 electoral college votes out of 538. So who is ahead?

Wisconsin - Biden

Arizona - Biden

North Carolina - Biden 

Georgia - Biden

Michigan - Biden

Ohio - Tied

Pennsylvania - Biden

Florida - Biden

Biden is trying to put Mr Trump on defence across the country and build an advantage in the Electoral College so large that the president might struggle to contest it.

READ MORE: US election polls: Joe Biden holds nine-point lead over Donald Trump

That is especially important since Mr Trump, who lost the popular vote in 2016, has said he may not accept the election results this year and has raised unfounded allegations that the increased use of mail-in voting amid the coronavirus pandemic could lead to fraud.

Mr Biden is complementing the expanded campaign travel with a late-stage ad push, reserving more than six million US dollars in television airtime in Texas, for decades deeply red, through the end of October, according to an Associated Press analysis of CMAG data.

He also plans to spend four million US dollars on advertising in Georgia, another Republican-leaning state that Democrats are feeling bullish about.

Mr Trump, meanwhile, has scaled back advertising in both states and has begun doing the same in Ohio, which he also won in 2016.

With Biden capitalising on having the campaign trail largely to himself by hitting critical swing states is his strategy paying off? We look at who is ahead in the key swing states.