Washington - Super Tuesday has never been bigger.
Republican presidential hopeful former Massachusetts Governor Mitt Romney particiaptes in the CNN and POLITICO.COM debate at the Ronald Reagan Presidential Library in Simi Valley, California, USA 30 January 2008. EPA/ANDREW GOMBERT
US candidates scrambling for presidential nominations are bidding Tuesday for support in the largest single day of party voting in US history, in a bewildering process in which each of 24 participating states sets its own rules.
The rules differ among the states and between the two major parties. In some states, only registered party members can vote, while others allow independents and even members of the other party to cross over.
Some states will hold conventional elections at polling stations, others will hold party caucus meetings in private homes, and one - West Virginia - will hold a state convention for elected Republican delegates.
The bottom line for the candidates is to send their slates of delegates to the national nominating conventions, held by the Democrats in August in Denver, Colorado, and by the Republicans in September in St Paul, Minnesota.
Usually, delegates must back the candidate for whom they were elected, at least through the first round of voting at the convention.
In a few places - notably New York, New Jersey and Missouri, some of Tuesday's biggest prizes - the Republican victor gains the state's entire delegation in a winner-take-all primary. The Democrats usually assign delegates proportionally.
At stake on Tuesday are 2,084 Democratic delegates and 1,108 Republicans, about half of the delegates to each convention. When the dust clears, about 30 of the 50 states will have voted in January or on Super Tuesday.
In addition to the grassroots delegates, each party has a contingent of elected officials and functionaries who also serve as uncommitted delegates, and given the closely contested races in each party, their votes could eventually tip the scales.
Democrats will send 4,049 delegates to their convention in Denver, Colorado, including 796 uncommitted 'super delegates' who include party officials, elected officials and dignitaries such as former president Bill Clinton. Democratic candidates Hillary Clinton and Barack Obama will be counting Tuesday night toward the 2,025-delegate majority that will give one of them the nomination.
Republicans will send 2,380 delegates including 463 officials to their convention. Frontrunner John McCain, chief rival Mitt Romney and dark horse Mike Huckabee will be eyeing the magic number of 1,191 delegates as Tuesday's results flood in from coast to coast.
Your Talkback on this Story