It’s called Super Tuesday for a reason.
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Today is the single biggest day of voting in the presidential race until November’s general election. Voters in 12 states and one territory will have a critical say in the course of the rest of the election.
Here’s a breakdown of everything you need to know:
History of 'Super Tuesday'
The phrase “Super Tuesday” was first used in the 1980s when Alabama, Georgia and Florida all held their primaries on the same day.
That number grew to nine four years later. That presidential election year, former Minnesota Sen. Walter Mondale survived the loss of seven states to his main opponent, but nabbed the eventual nomination at the Democratic National Convention.
In 1988, the foundation for today’s Super Tuesday was created, Southern Super Tuesday, and included mostly Southern states. Until 2004, these Southern primaries were held the week following Super Tuesday.
Today, the day when the greatest number of states hold primary elections has been dubbed the “SEC Primary” because many of the states that vote are represented in the Southeastern Conference (SEC), a U.S. college athletic conference.
Which states will be voting?
Alabama, Arkansas, Georgia, Massachusetts, Minnesota, Oklahoma, Tennessee, Texas, Vermont and Virginia will hold primaries for both parties. Alaska will host only a Republican primary; American Samoa, only a Democratic one.
In Colorado, there will be caucuses for both parties. But only the Democrats in that state will be choosing their candidates. Colorado Republicans have opted to vote for the delegates who will represent them at the Republican National Convention in July.
What is a delegate and how many delegates are at stake on Super Tuesday?
Delegates are selected from each state to cast their votes for a presidential nominee at the Democratic and Republican National Conventions in July. The number of delegates and how they are selected varies from state to state.
There are two kinds of delegates: pledged, who must vote based on primary and caucus results; and unpledged, or super, who are permanent delegates and can vote for whom they please at the conventions.
A Republican candidate must receive 1,237 delegates out of the total 2,472 available to be eligible for the nomination. For the Democrats, that number is 2,383 out of the total 4,765 delegates.
There are 595 delegates at stake for the Republicans today, about 20 percent of the total GOP delegates. An 1,015 delegates are up for grabs for the Democrats.
How many delegates do the candidates have so far?
There have been election events for both parties in four states: Iowa, New Hampshire, Nevada and South Carolina.
Super Tuesday VisitsBefore the results rolled in during the South Carolina primary last month, Democratic presidential candidate Bernie Sanders was already on to the Super Tuesday States, but he wasn’t the only one. Republican presidential candidate and Ohio Gov. John Kasich hit the trail for Georgia as supporters were casting their ballots in the Nevada caucuses last month. So, just how many times did each candidate pay a Super Tuesday state a visit? Let’s take a look.
Donald Trump: 27 Ted Cruz: 35 Marco Rubio: 18 John Kasich: 18 Ben Carson: 26
And on the other side of the aisle,
Hillary Clinton: 25 Bernie Sanders: 24
*does not include number of stops made in each state or visits for interviews-fundraising.
Who’s leading where?
The New York real estate mogul, Donald Trump, is leading the GOP field in Tennessee and Georgia, and Texas Sen. Ted Cruz is leading in his home state, according to new polls released Sunday. In the latest Quinnipiac polls, Trump is also shown holding a lead in Florida, the home state of main GOP rival Sen. Marco Rubio, and Ohio, the home state of Gov. John Kasich. The pressure is on for Cruz, Rubio and Kasich to come out on top in their home states, where a potential loss could go as far as costing them their campaign.
Who can vote?
Voting rules vary based on the state’s kind of election. Alabama, Arkansas, Georgia, Tennessee, Texas, Vermont and Virginia all have open primaries. This means that a registered voter in these states, regardless of their party registration, can cast their ballot for a candidate in either party.
Oklahoma has a closed primary and American Samoa, Alaska, Colorado and Minnesota all have closed caucuses. Voters in these states must vote for a candidate within their registered party. Massachusetts has a “modified closed” primary where registered Republicans and Democrats must vote in their own parties, but independents can choose to affiliate with either party once they get to the polls. Democrats living abroad may also vote in their “global presidential primary” beginning on Super Tuesday.
What Happened Last Time?In 2008, the last election without an incumbent candidate, Super Tuesday was a decisive victory for Sen. John McCain, who would go on to become the GOP nominee. In the Democratic race, Super Tuesday did not provide a clear victory for either Hillary Clinton or Barack Obama. Although Obama won more states overall, Clinton won states with higher numbers of delegates.
ABC News’ Alana Abramson contributed to this report.