Today is Super Tuesday, the biggest single day of the Democratic primary season.
Voters will cast their ballots in 14 US states and in the US territory of American Samoa.
Democrats abroad will also begin voting.
The early states of Iowa, New Hampshire, Nevada and South Carolina were important for candidate momentum and profile but now it's all about delegates.
At least 1,991 pledged delegates are needed to win the Democratic nomination on the first ballot at the party's convention in July.
Today, there are a massive 1,357 delegates up for grabs, that's more than a third of the total delegate count.
Big states will be voting like California, where 415 pledged delegates are available, and Texas with its 228 delegates.
To put those numbers in context, only around 150 delegates have so far been allocated.
The Vermont senator Bernie Sanders is leading in the delegate count, but only just.
Joe Biden isn't far behind thanks to his convincing win in South Carolina on Saturday night.
Mr Biden will hope that his victory at the weekend will give him a boost on Super Tuesday.
He won the South Carolina primary thanks to strong support from the African-American community.
If that trend continues, Mr Biden should do well in Super Tuesday states with large black populations such as Tennessee, Alabama, North Carolina and Virginia.
But when it comes to the two biggest states, California and Texas, Mr Sanders is ahead in the polls.
A progressive candidate to the left of the Democratic Party, he has become the frontrunner because there were so many moderates running on the other side, splitting the vote.
There needed to be drop outs so that centrist voters could unite behind one candidate to take on Mr Sanders.
That finally started happening in recent days.
First, the billionaire businessman Tom Steyer suspended his campaign on Saturday night after coming third in the South Carolina primary.
Next it was the turn of Pete Buttigieg to pull out of the race and then last night Senator Amy Klobuchar announced the end of her campaign.
They went further, with Mr Buttigieg, Ms Klobuchar and former candidate Beto O’Rourke all endorsing Mr Biden last night.
It is starting to look like a two-horse, Bernie-Biden race but not quite.
The big question is, how will Michael Bloomberg do?
The billionaire former mayor of New York skipped the early voting states and will be on the ballot for the first time today having pumped hundreds of millions of dollars of his own money into campaigning in Super Tuesday states.
It's a big political gamble and he will find out in the coming hours if it has paid off.
