It's Super Tuesday — the biggest day in the Democratic presidential primary calendar when 14 states vote in every region of the country to decide who will face off against Donald Trump in November.
The outcome of today's context won't necessarily determine who wins the nomination, but it has historically been difficult for a candidate who performs poorly on the day to recover.
In particular, California and Texas provide the two biggest delegate hauls of the entire primary season (415 delegates and 228 respectively).
The other states voting are Arkansas, Alabama, Colorado, Maine, Minnesota, Massachusetts, Vermont, Utah, North Carolina, Virginia, Tennessee and Oklahoma.
In California and Texas, white voters make up less than half the population. Latinos count for nearly 40% of the populations in both states. California, meanwhile, has the nation's highest Asian population, at roughly 15%. North Carolina, Tennessee, Virginia, Alabama and Arkansas have large populations of black voters, one of the Democratic Party's core voting blocs.
The states blend a mix of urban and rural voters, as well as Democrats from all sides of the political spectrum. As candidates focus on wooing Southern Democrats in states like Texas and Arkansas, they must also be thinking about a message that can resonate in progressive San Francisco.
Bernie Sanders and former New York Mayor Mike Bloomberg have had the widest and most aggressive footprint across the Super Tuesday states, largely because they've got the money to spend.
Primaries and caucuses
Although third parties can have candidates, the main race is between the left-wing Democratic Party and the right-wing Republican Party.
The parties elect their candidates through primaries or caucuses in each of the states and US territories.
Candidates are aiming to win states by winning delegates, who are assigned locally to represent the voters and nominate the candidate at the later convention.
Most states hold a primary election, funded by the state government. This is where voters go to a polling place (normally open all day) to cast a secret ballot, said University of Delaware professor David Redlawsk.
The primary can be open or closed, meaning that in certain states, voters that do not belong to a specific party can vote in that party's primary.
After the early states narrow down the Democratic field, several states will hold primary elections all at once, Super Tuesday.
"About 40% of all delegates will be elected for the Democrats on Super Tuesday, which means a lot will be at stake," said Redlawsk.
There are still several states that will hold primary elections through June when the contest can remain wide open, For example, the Associated Press reported that Hillary Clinton would be the Democratic candidate as late as June 2016. Trump
Parties officially nominate their candidates in a convention - a large gathering in which the candidate gives a speech and the delegates officially cast their votes for the candidates.
This year the Democrats will hold their convention in July and the Republicans hold their conventions in April. This marks the end of the primary season as the national campaign begins for the official candidates of each party.
The election will take place on November 3, 2020, where voters cast their ballot in their home state. Each state has a select number of electors.
Presidential candidates have to win at least 270 of the 538 total electors in order to win the race.