Guardian US polling expert Harry J. Enten breaks down the workings of the Electoral College – a mysterious institution that uses the people's votes to select the US president. With 538 electors up for grabs across 50 states and the District of Columbia, Barack Obama and Mitt Romney are vying for the 270 votes that will grant them the White House. It's a curious system where things can and have gone awry – one possible outcome in 2012 is for Romney to end up in the Oval Office with Joe Biden as his vice-president. How could that happen? Let Harry explain.