In 23 caucuses around Wyoming, Democrats, who make up about 25  per cent of the staunchly Republican state's electorate, chose between Clinton or Obama.

Officials reported large turnout in a state known for its Republican sympathies.

"I didn't know there was this many Democrats in the whole  state," said Dick Sadler, a party official.

 

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After Wyoming, Obama and Clinton will face voters in the southern state of Mississippi, where 33 delegates are at stake.

 

The campaign will then move to Pennsylvania on April 22.

Obama is favoured to win Mississippi.