Mitt Romney, the former Massachusetts governor, is just ahead of John McCain in Nevada's Republican race, polls indicate.
In Nevada, Hillary Clinton's campaign criticised a judge's decision on Thursday to permit casino workers to caucus in special precincts in the city of Las Vegas, saying the move was unfairly biased towards other candidates.
However, supporters of Barack Obama, who benefits from the ruling as he was endorsed by the union representing many of the workers, welcomed the decision.
The ruling followed a lawsuit brought by the western state’s main teachers' union, which had endorsed Clinton.
Meanwhile, in South Carolina supporters of Arizona senator McCain have condemned an apparent smear campaign in which fliers were sent to voters.
The fliers said McCain had betrayed fellow prisoners of war to save himself while being held captive in Vietnam during the war.
In 2000 McCain lost the Republican presidential nomination to George Bush, a loss some blamed on a smear campaign in the same state which implied he had fathered a black child out of wedlock - a reference to his adopted daughter from Bangladesh.
|Romney has a very small lead in Nevada [AFP]|
McCain, although leading in many polls in South Carolina, is facing competition from Mike Huckabee, who has appealed to the southern state's evangelical Christian voters.
Romney trails both in third place.
Obama is the favoured Democratic candidate to win the state, which will hold its Democratic primary on January 26.
After South Carolina and Nevada the candidates will gear up for Florida and so-called "Super Tuesday" on February 5, when more than 20 states will hold primaries or caucuses simultaneously.