The three candidates for the Democratic nomination in the US presidential race are taking part in a TV debate, as the contest appears to be getting tighter.

While Hillary Clinton leads nationwide, she is facing a threat from Vermont senator Bernie Sanders in key states.

Hours before the debate in Charleston, South Carolina, Mr Sanders unveiled a plan for healthcare for all American citizens.

This is the final Democratic debate before caucuses in Iowa on 1 February.

Former Maryland Governor Martin O’Malley, who is trailing Ms Clinton and Mr Sanders in polls, is also taking part.

Mr Sanders’ universal healthcare plan, announced two hours before the debate started, would see citizens pay what he called “a 2.2% income-based premium” towards healthcare. Companies would pay an extra 6.2% of an employee’s income towards the plan.

The plan, Mr Sanders’ campaign website said, was a way to “stop forcing working Americans to choose between bargaining for higher wages or better health insurance”.

Ms Clinton had previously indicated that moves planned by Mr Sanders could “strip millions and millions and millions of people off their health insurance”.

The debate is being held near a church where nine parishioners were shot dead in June 2015 and gun control is expected to be on the agenda.

Ms Clinton released an advertisement this week attacking Mr Sanders for his attitude towards gun control. His home state, Vermont, has one of the highest rates of gun ownership in the US, with close to one in two households owning a weapon.

The ad noted Mr Sanders’ support for a 2005 law giving gun manufacturers legal immunity. He says he backed the law as it put into place tighter restrictions on gun production.

Mr Sanders has accused his rival of being weak on Wall Street banks. He says he would break up large banks.

Polls indicate Ms Clinton and Mr Sanders are neck-and-neck ahead of the caucus in Iowa, where voters will decide who they want as their preferred candidate. She had once commanded a large lead.

In New Hampshire, Mr Sanders holds a lead in polls ahead of voting in the primary there eight days later.

Q&A: US primaries and caucuses