The keys to Super Tuesday
It is the most important day of the presidential primary elections: it decides 865 Republican delegates (36% of the total at stake) in 15 states and 1,420 Democratic delegates in 15 states and the territory of Samoa.
Before Tuesday, former President Donald Trump had 276 delegates, compared to 43 for former Governor Nikki Haley.
In total there are 2,429 at stake, so whoever secures 1,215 will be the Republican candidate.
The president, Joe Biden, had 206 delegates before Tuesday and has no major rival, so this election measures his strength against the protest vote.
Primaries for the Senate, House of Representatives, governor and other local offices are held in California, Texas, North Carolina, Alabama and Arkansas.
The first polls opened in Vermont at 6 a.m. ET.
The last ones close in Alaska at midnight (Eastern time).