In a matter of weeks, former vice-president Joe Biden lost his status as the front runner for the Democratic nomination. That title now belongs to Sen. Bernie Sanders — and could be written in stone after Tuesday's string of primaries across the United States.

But first, Saturday's South Carolina primary will give Biden one last chance to make himself a contender again.

The next few days will be decisive in the Democratic primaries. Only 2.5 per cent of the 3,979 pledged delegates that will head to the Democratic National Convention in July have been awarded so far.

The South Carolina primary will add 54 delegates to the total, but the big prize will come on Super Tuesday, when 14 states and American Samoa vote. After the results are in, nearly 38 per cent of delegates will have been allocated.

Up to now, the three votes that have been held in Iowa, New Hampshire and Nevada have been more important for setting the narrative for the campaign than for their contributions to the delegate count.

Biden's poor performances in the first two states knocked him from the top of the polls. A tie in Iowa, a close win in New Hampshire and a big victory in Nevada resulted in a Sanders surge.

In polls conducted at least partly after the Nevada caucuses, Sanders is averaging 29 per cent support nationwide. That puts him well ahead of Biden, who trails with an average of 18 per cent support among Democratic primary voters.

Former New York City mayor Mike Bloomberg, who has yet to contest a state in the race, trails in third with 16 per cent, followed by Sen. Elizabeth Warren and former South Bend, Indiana mayor Pete Buttigieg at 11 per cent each.

Sen. Amy Klobuchar, billionaire Tom Steyer and Rep. Tulsi Gabbard follow with between two and four per cent apiece.

Sanders' momentum appears to have continued post-Nevada. He's picked up an average of three points since polls conducted before the state's caucuses. Biden, however, appears to have halted his decline; though he has dropped significantly from where he stood in January before the first ballots were cast, he has held steady in the polls over the last week or so. Instead, Warren and Klobuchar have dropped slightly.

But while he holds a sizeable national lead over Biden, Sanders is not yet in a dominant position. That leaves the door open to a contested convention in July if Sanders fails to secure a majority of delegates by then. What happens in South Carolina could have a bearing on that.

Biden's turn for a big win in South Carolina

With his strong support among African American voters, Biden was always counting on South Carolina to give him a boost. What he wasn't counting on was a fourth-place finish in Iowa and an embarrassing fifth-place showing in New Hampshire. That made his second-place result in Nevada look good by comparison, even though Sanders had more than twice as much support as him.

He desperately needs a win in South Carolina. The polls suggest he's likely to get one.

Surveys conducted in the last week give Biden a big lead, with 35 per cent support on average to just 19 per cent for Sanders. Trailing in third with 14 per cent support is Steyer, who has spent millions of dollars of his own money on campaign ads in the state (he did the same in Nevada, though that didn't help him crack double-digits in support).

After spending millions on ads in the state, billionaire Tom Steyer is polling third in South Carolina. Nationwide, however, he is averaging just 2.5 per cent support. (Gerald Herbert / Associated Press)

No other candidate is averaging over nine per cent support in South Carolina. As the rules of the Democratic primaries only award delegates to candidates who achieve at least 15 per cent support statewide (or in congressional districts), that might mean only Biden, Sanders and potentially Steyer could come out of South Carolina with any new pledged delegates.

Biden does seem to have turned around his campaign in the state. Polls conducted before the Nevada caucuses but after Iowa voted had put Biden and Sanders in a close race, with only four percentage points separating the two. Now, the gap is 16 points.

This is in large part due to Biden's support among African American voters, who make up about 60 per cent of the Democratic primary electorate in South Carolina. An average of three recent polls with demographic breakdowns gave Biden 43 per cent support among those voters, more than double Sanders' 19 per cent.

A big win could certainly give Biden's national poll numbers a jolt — results in the three states that have voted have had an obvious and significant impact on the polls. But only three days separate South Carolina's primary from Super Tuesday. Is that enough time for Biden to turn his campaign's negative trajectory on its head?

Sanders poised to rack up delegates on Super Tuesday

The latest polls suggest that Sanders, not Biden, is likely to do quite well next week. Among states with polling available, Sanders is leading or is in contention in eight of them, compared to just four for Biden, three for Bloomberg and one apiece for Warren and Klobuchar.

While Sanders is leading in small states like Vermont and Maine that are in his own backyard, he also has a sizeable lead in California — the biggest prize, with 415 delegates — and is ahead in Virginia and Colorado. He appears to be in a close race with Biden in Texas and with Warren in her home state of Massachusetts, while the latest numbers suggest he is in a three-cornered contest with Biden and Bloomberg in North Carolina.

Mike Bloomberg, left, is not contesting South Carolina but could win lots of delegates on Super Tuesday. Pete Buttigieg, centre, and Elizabeth Warren, right, are not poised to do as well, according to the polls. (Matt Rourke / Associated Press)

Sanders is likely to secure delegates in every state. No other candidate can claim the same thing. Both Biden and Bloomberg might do well in the South. Klobuchar might win her home state of Minnesota and Buttigieg might be able to win delegates in a handful of states.

But without a big shift in the next few days, it will be difficult for any candidate to catch Sanders. In Iowa, New Hampshire and Nevada, Sanders averaged just 28 per cent of the popular vote but 45 per cent of the delegates. That is still not a majority, but if he keeps that rate of success up for the rest of the primaries, it will be difficult to deny him the nomination at the convention, even if he doesn't have a majority.

And, as primaries have often shown, voters like backing a winner. So far, that's what Sanders looks like. On Saturday, Biden finally gets his shot to be one — for now.