Former Arkansas Governor Mike Huckabee has won the small West Virginia caucus/convention, the first result of Super Tuesday.
24 US states are holding elections on what has been dubbed 'Super Duper Tuesday' with the last polls due to close at 4am tomorrow morning.
Mr Huckabee's victory came after John McCain supporters switched sides in a second round of voting in order to avoid handing Mitt Romney the state's 18 delegates.
Senator McCain holds a commanding poll lead in the Republican race while the Democratic race is evenly split between Senators Hillary Clinton, who cast her vote in Chappaqua, New York this morning, and Barack Obama, who was in Chicago, Illinois.
Today's round of voting includes more states than ever before and is taking place earlier than in previous contests.
It is being called a national primary, because this Super Tuesday is unlike anything that has ever happened before in US electoral history.
States from New York to North Dakota, Kansas to California will vote in a combination of primaries and caucuses for the nominees of the Republican and Democratic parties in this year's presidential election.
Despite Huckabee's victory, polls indicate the Republican race could be decided today with Senator McCain enjoying a clear lead. His closest rival Mitt Romney is polling well in California, the biggest state on the electoral map in terms of delegates.
On the Democratic side, Mrs Clinton and Mr Obama are in a virtual tie.
Many analysts believe the high number postal ballots in California and the closeness of the race could mean it will be very late before the all-important state is declared.
Dublin pub hosts Democratic vote
The Dublin pub O'Neill's was turned into a voting centre today so US Democrats living here could cast their vote for the party's Presidential nominee.
The Democrats Abroad Global Primary runs from today until 12 February in more than 30 countries around the world.
The organisation will actually be represented by delegates at the Democratic convention this August in Denver, Colorado.
Click here to visit RTÉ.ie's US election site.