Americans vote on Tuesday in what is deemed the most pivotal day in the presidential nominating process, with front-runners Hillary Clinton and Donald Trump hoping to finish off their challengers.
Voters in a dozen states will take part in "Super Tuesday" — a series of primaries and caucuses in a dozen states ranging from Alaska to Virginia.
If Democrat Clinton and Republican Trump — an outspoken billionaire political neophyte who has unexpectedly tapped into a reservoir of conservative rage at conventional politics — win big, it could spell doom for their challengers.
With just hours to go before polls open, the duo made last-ditch appeals to supporters ahead of a day like few others on the calendar leading to the November election for the White House.
Trump's Republican rivals, Senators Marco Rubio and Ted Cruz, were trying frantically to halt the real estate magnate's march toward the nomination, seeking to unite the party against the man they see as a non-conservative political interloper.
Clinton meanwhile was riding high after thrashing rival Bernie Sanders in South Carolina over the weekend, securing an astronomical 86 percent of the African-American vote in her third win in four contests.
Here is what is at stake on "Super Tuesday," which could have a big impact on Democratic and Republican contenders still in the race for the White House:
Twelve states vote
A dozen states are holding contests, including several in the South. For the most part, they are both Republican and Democratic in nature. Two exceptions are Alaska, where only Republican caucuses are being held, and Colorado, where only Democratic caucuses will take place.
Democrats will be voting in 11 states and American Samoa, with 865 delegates at stake. Democrats abroad are allocated 17 of those delegates. Republicans, meanwhile, will be voting in 11 states on Super Tuesday.
In contrast to primary elections, caucuses are meetings which voters attend and where they indicate their preference.
Here is the list of "Super Tuesday" states in alphabetical order:
Alaska (northeast, Republican caucus)
Colorado (west, Democratic caucus)
Democrats are also voting in American Samoa in the Pacific.
Delegates in play
Around a quarter of all delegates doled out during the primary process will be up for grabs.
Texas has the largest number of delegates in play (222 on the Democratic side and 155 on the Republican side.) Alaska and Vermont have the fewest.
Primaries and caucuses serve to elect delegates to national Democratic and Republican conventions where a party's presidential candidate is chosen ahead of the November elections.
The Republican National Convention will be held 18-21 July in Cleveland, Ohio. The Democratic National Convention takes place in Philadelphia, Pennsylvania, 25-28 July.
For Republicans, the first candidate to reach 1,237 delegates out of 2,472 will get the party's nomination.
On the Democratic side, the support of at least 2,382 delegates out of 4,763 is needed.
With inputs from agencies