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 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.