As the Democratic field shrinks, so do the chances of a contested convention

To avoid a contested convention and block the path of Sen. Bernie Sanders to the Democratic U.S. presidential nomination, the field of contenders needed to get smaller and the vote of Democratic moderates had to coalesce behind one candidate.

In less than a week, both of those requirements have been fulfilled.

Former vice-president Joe Biden's stunning performance in the Super Tuesday primaries not only resurrected his foundering third attempt for the U.S. presidency, it made it all the more likely that the nomination will not have to be decided on the floor of the Democratic National Convention in July.

The wheels were set in motion on Saturday when Biden crushed his rivals in the South Carolina primary. The disappointing showings for Sen. Amy Klobuchar and former South Bend, Ind., mayor Pete Buttigieg led them to withdraw from the race and endorse Biden.

Their exits were a boon for him. According to one poll, Biden took the lion's share of their orphaned voters, jumping 10 percentage points in national support. The surge helped him win big on Super Tuesday, carrying most of the states up for grabs and a few that were thought out of reach for him.

Former New York City mayor Mike Bloomberg, who spent about half-a-billion dollars on his campaign, saw little return for his investment. After failing to secure a large number of delegates, Bloomberg suspended his bid and also threw his support behind Biden.

That move is likely to be as beneficial to Biden as the endorsements of Buttigieg and Klobuchar — and potentially more beneficial, if Bloomberg puts his formidable financial resources behind Biden's campaign.

The delegate math just got easier

The main reason that a contested convention is now less likely is a simple matter of math. The Democrats award delegates proportionately to candidates who reach the threshold of 15 per cent support in each individual state or congressional districts. The more candidates who reach that threshold, the more delegates they have to divvy up and the harder it is for any candidate to win a majority.

Complete results from Super Tuesday won't be known for some time yet (some mail-in ballots, for example, still have to be counted). But estimates by the New York Times Wednesday put Biden at 670 delegates, followed by Sanders at 589. Bloomberg and Sen. Elizabeth Warren had about 100 delegates apiece. Buttigieg and Klobuchar won 33 delegates earlier in Iowa and New Hampshire, while Rep. Tulsi Gabbard secured a single delegate in the American Samoa caucuses on Tuesday.

This estimate could be off by a few dozen delegates in one direction or the other, but it gives us something to work with.

Candidates need 1,991 pledged delegates to win a majority on the first ballot of the Democratic convention. After this week, that means Biden and Sanders will be about 1,300 to 1,400 delegates short.

But there are a lot of delegates still to be won, with just under 2,500 yet to be awarded in the states and territories that will vote between now and early June. Based on the New York Times estimate, that means Biden would need to win a little over 53 per cent of remaining delegates, while Sanders would need to win about 56.5 per cent.

In a race with four or more viable candidates, that would be a tough mark to hit. Before Super Tuesday, Sanders was the delegate leader with less than 40 per cent of the total. In a two-horse race, however, that's achievable. In the 2016 Democratic primaries, Hillary Clinton won about 60 per cent of the delegates in her head-to-head match-up with Sanders.

Whither Warren's voters?

The road ahead looks tougher for Sanders than Biden. Polls suggest that Biden was already riding a wave of positive momentum going into Super Tuesday. The results of that vote are unlikely to do anything but add to that momentum and nearly all of the big states remaining on the calendar are ones that Sanders lost to Clinton in 2016.

But the campaign has been a roller coaster to date. There's no reason to believe that it can't swing again.

The next potential inflection point — at least ahead of next Tuesday's primaries in a few key states, including Michigan, Washington and Missouri — could be the repercussions of Warren's decision on Thursday to drop out.

Reports emerged on Wednesday that she was mulling her options. The Super Tuesday results certainly gave her much to think about. Warren didn't carry a single state, placed third in Massachusetts and, at last count, was fourth in Oklahoma (where she was born and raised).

The 200 or so delegates won by candidates who have withdrawn from the race would pose just a small obstacle at the convention. Only if Biden and Sanders finish within 200 delegates (or five per cent) of each other would either of them fail to win a majority before the convention.

Warren dropping out is likely to help Sanders — and he certainly needs her voters to beat Biden. Exit polls show her support is strongest among young voters and those who say they are "very liberal" — the same electorate that Sanders dominates. Polls also suggest that Sanders is the second choice of Warren voters over Biden by a significant margin. 

Much like the help Biden got from the exit of Buttigieg and Klobuchar (and potentially Bloomberg), it could be Sanders' turn to benefit from the last major candidate throwing in the towel.

So, the contest is by no means over yet. But it has shifted significantly. From front-runner to also-ran and back again, Biden could make this nomination his to lose with another good showing at the polls next week.

Then he just needs to make sure he doesn't lose it again.