What is Super Tuesday? Why it matters and what to watch

The biggest day of this year's primary campaign is approaching as 15 states — plus American Samoa — vote in contests known as Super Tuesday.

The elections are a crucial moment for President Joe Biden and former President Donald Trump, who are the overwhelming front-runners for the Democratic and Republican presidential nominations. As the day with the most delegates up for stake, strong performances by Biden and Trump would move them much closer to becoming their party's standard bearers again, setting up a likely rematch between the two in November.

Here are some things to watch: