When the richest haul in the US presidential primary race arrives on "Super Tuesday," millions of Americans will cast their votes -- and many of them will count for nothing.

In the primary system it is the number of allocated delegates that matters more than the popular vote count.

The Republican and Democratic nominating contests that play out across the United States every four years serve to elect delegates to the national conventions, where they formally select their party's presidential nominee.

Most of the 15 states holding primaries on "Super Tuesday" award them on a winner-takes-all basis, meaning a strong second place showing can result in zero delegates for that candidate.

A supporter holds a placard as US Republican presidential hopeful and former UN Ambassador Nikki Haley speaks during a campaign rally in Portland, Maine, on March 3, 2024.

It's a system that tends to favor front-runners, and for Nikki Haley, the sole remaining challenger to Donald Trump for the Republican nomination, it's one that has dulled her already razor thin chances.

Haley won 40 per cent of the vote in her home state of South Carolina, but received only three delegates, while Trump added 47 to his seemingly unassailable tally.

On Sunday night Haley chalked up her first primary win, in Washington, DC, taking 19 delegates.

 Nominees decided by March? 

With 2,429 Republican delegates from across the country, Trump needs at least 1,215 to secure his lock on the nomination.

Having won all eight early state and territory contests, he already has more than 247 delegates in his column compared to Haley's 24 going into Super Tuesday, when more than a third of the national Republican total is up for grabs.

Former US President and 2024 presidential hopeful Donald Trump speaks during a "Get Out the Vote" rally at the Greater Richmond Convention Center in Richmond, Virginia, on March 2, 2024.

Trump's dominance is such that he could cross the magic threshold as early as March 19 -- even though the primary calendar stretches into early June.

On the Democratic side, incumbent Biden is a virtual lock.

The delegate numbers differ between the parties, but the process is similar. There are currently 3,934 Democratic delegates in play, and Biden's magic number for victory is 1,968. He already has 206 delegates, and could also secure the nomination in March.

That means the re-match between Biden and Trump -- who will be the oldest ever pair of nominees -- will involve one of the longest presidential campaigns in US history.

The Republican National Convention is scheduled for July in Milwaukee, Wisconsin, while Democrats gather for their convention in August in Chicago.

The delegates also endorse the party's candidate for vice president, selected by the presidential hopeful and often announced a few days before the convention.