Voting has begun in several US states in the biggest day of nominations in the race for US president.



Known as Super Tuesday, the vote involves 24 states accounting for over half the number of delegates who will later select candidates at the party conventions.



The Republican race, in which John McCain is leading his main challenger, Mitt Romney, may be decided today.



Early reports on the Democratic race suggest that rivals SenatorHillary Clinton and Senator Barack Obama are so close that this contest may go further.