Whilst the 2016 US presidential election won't be held until later this year, the race to be the Democrat candidate on the ballot box is already taking shape.

Heading into the Iowa caucus on 1 February, the first of the 50 states to nominate which candidate they want to represent their respective party, there are three main challengers looking to succeed Barack Obama as president of the US: Hillary Clinton, Bernie Sanders and Martin O'Malley.

Clinton is the best known and favourite of the three. The former First Lady and wife of then president Bill Clinton, the 68-year-old is a veteran politician well known to the American public.

But whilst the former secretary of state has long been seen as the main contender, she is facing a real battle against surprise package Bernie Sanders. The 74-year-old Vermont senator has astonished observers by securing widespread grassroots support on the back of his socialist campaign.

The third candidate on the list is Martin O'Malley. The 53-year-old former governor of Maryland has pulled off surprise victories before in his political career, but at the moment is struggling to get his voice heard.

In this video, IBTimes UK give a guide to each candidate, along with their respective strengths and weaknesses as they look to secure their party's nomination.

Read our guide to the Republican candidates here

US election 2016: Who are the main Republican candidates? IBTimes UK