Voters in a dozen states and one U.S. territory are set to head to the polls Tuesday to vote for who they think should be the nominee of their party. During Super Tuesday, the unofficial name for the event, the largest number of states hold their primaries or caucuses. 

Alabama, Arkansas, Georgia, Massachusetts, Minnesota, Oklahoma, Tennessee, Texas, Vermont and Virginia will hold the voting events for both Republicans and Democrats Tuesday. Republicans will also hold caucuses in Alaska, while Democrats will hold their their caucuses in Colorado. Democrats will also vote in American Samoa, a U.S. territory in the South Pacific.

Candidates rely on Super Tuesday to keep momentum going during their campaign. At stake are 661 Republican and 865 Democratic delegates. The event was originally conceived in 1988, in part because Southern Democrats wanted to emphasize the importance of their region, Politico reported. The event also forces candidates to campaign nationwide rather than focus on securing the nomination in a specific state.

Heading into Super Tuesday, real estate mogul Donald Trump continues to lead in most polls on the Republican side. Vying for second are Florida Sen. Marco Rubio and Texas Sen. Ted Cruz. Ohio Gov. John Kasich and retired neurosurgeon Ben Carson have kept their campaigns going, despite having experienced difficulty building excitement. It is likely the two could face pressure to drop out of the race so that the GOP can unify behind a candidate to beat Trump.

On the Democratic side, former Secretary of State Hillary Clinton continues to lead, but Vermont Sen. Bernie Sanders has proved a serious contender. Although Clinton began her campaign as the presumed nominee last year, Sanders, a longtime independent who is running on the Democratic ticket, has seen support surge, particularly among young voters.

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