Turnout key in Iowa caucuses as Donald Trump and Hillary Clinton lead the polls

Democratic and Republican presidential candidates across Iowa are scrambling for the first votes in the 2016 race for the White House, urging their supporters to take part in today’s caucuses in which outsiders Bernie Sanders and Donald Trump are pinning their hopes on a large turnout.

The caucuses will provide a big test of whether the large enthusiastic crowds attending rallies for Mr Trump and Mr Sanders will turn into actual votes when Iowans gather on a wintry night for meetings at schools, libraries and even private homes in the first in a series of state-by-state nominating contests.