Polling shows that only Mr. Sanders is positioned to break the threshold across all the states and in most districts. The more candidates who don’t clear 15 percent, the better the night will be for him. But the more places where multiple candidates hit that 15 percent mark, the more it will cut into Mr. Sanders’s expected delegate lead.
Here’s how this could play out.
Let’s pick a congressional district: California’s 19th, which includes San Jose. It awards six delegates.
And let’s say Mr. Sanders, Mr. Biden and Ms. Warren all break the threshold. The delegates would be split proportionally among them, meaning Mr. Sanders would get, at most, four delegates out of the district. But if only Mr. Sanders breaks 15 percent, he captures all six delegates.
Now, multiply that kind of math by the whole state of California, which has 415 pledged delegates — 271 apportioned based on the results in the state’s 53 congressional districts and 144 awarded by statewide vote — and you can see how those small gains really start to add up.
In the contests so far, Mr. Sanders has benefited from the fact that many of his rivals failed to break that 15 percent threshold. In Iowa and New Hampshire, he captured about a quarter of the vote. In South Carolina, he won about 20 percent. Yet he still leads the delegate race, with 39 percent of the delegates awarded so far.
Now, with Mr. Buttigieg and Ms. Klobuchar out of the race, the question is whether their supporters might help put Mr. Biden, Ms. Warren and Mr. Bloomberg over that 15 percent mark in enough states and districts to prevent Mr. Sanders from building an insurmountable lead.
It’s unclear whether the voters backing Mr. Buttigieg and Ms. Klobuchar will follow their lead and flip to Mr. Biden. Some of their supporters presumably cast their ballots already, given that several million early votes have been cast across the Super Tuesday states, according to Democratic strategists. And voters don’t always stay within ideological lanes: At least one poll shows that Mr. Sanders is the most popular second choice of Buttigieg backers.