For all the signs of party influence in Biden's comeback, “there was a lot of contingency,” says Matt Grossmann, an associate professor of political science at Michigan State University. “I don’t think it was inevitable.”

There has long been tension between pure democracy and the preferences of political elites in America’s nomination system. Both played a part lately.

But one foundational aspect of Mr. Biden’s sudden strength was almost certainly the role played by the institutional Democratic Party. Party actors from elected officials to local activists decided within a few days to coalesce around an imperfect but broadly acceptable alternative to Sen. Bernie Sanders, whom many feared would drag them to defeat in down-ballot races this fall.

In part it was geographic – the former vice president’s core supporters were scarce in early voting states, but more prevalent from South Carolina on. He also likely benefited from a bandwagon effect, attracting support because suddenly he seemed like a winner.

Two weeks ago, Joe Biden’s presidential candidacy was almost dead. Today he looks like the presumptive nominee of the Democratic Party. What happened?

Two weeks ago, Joe Biden’s presidential candidacy was almost dead. Today he looks like the presumptive nominee of the Democratic Party. What happened?

In part it was geographic. The former vice president’s core supporters were scarce in early voting states, but more prevalent from South Carolina on. He also likely benefited from a bandwagon effect, attracting support because suddenly he seemed like a winner.

But one foundational aspect of Mr. Biden’s sudden strength was almost certainly that much of the institutional Democratic Party stood up and rallied around him. After rank-and-file African American voters threw their weight behind him in South Carolina, party actors from elected officials to local activists decided within a few days to coalesce around an imperfect but broadly acceptable alternative to Sen. Bernie Sanders, whom many feared would drag them to defeat in down-ballot races this fall.

Sen. Amy Klobuchar and Pete Buttigieg dropped out of the race and endorsed Mr. Biden just prior to Super Tuesday. Former rival Beto O’Rourke backed him, too. Money and more endorsements began flooding in. Suddenly, the way was clear for voters to unify around a single alternative to Senator Sanders – and they did.

Sanders supporters have complained that the playing field isn’t level and that party coordination of any sort is an undemocratic influence. But this isn’t a new conflict. Tension between pure democracy and the preferences of political elites has long been a part of America’s complicated presidential nomination system.

In 2020, it took so long it almost seemed it wouldn’t happen.

“But given the situation, I think the party has acted like it’s deciding,” says Hans Noel, an associate professor of government at Georgetown University and co-author of the seminal 2008 book “The Party Decides: Presidential Nominations Before and After Reform.”

The results of this week’s “mini-Tuesday” of six state primaries haven’t brought an official end to the Democratic nomination contest, of course. Mr. Sanders on Wednesday vowed to stay in the race, citing the popularity of his proposals and his appeal to younger voters. He said he planned to attend a scheduled Sunday Democratic debate in Phoenix.

But barring a major unforeseen development, the Vermont senator’s path to the nomination seems all but gone. Mr. Biden won four states on Tuesday, including Michigan, the day’s biggest prize. Florida, Ohio, Illinois, and Arizona will hold primaries next week, and the former vice president is heavily favored in all.

Given that Democratic primaries award delegates proportionally, Mr. Biden will likely soon have a delegate lead that for all practical purposes is insurmountable, if he doesn’t already. On Wednesday afternoon, the data journalism site FiveThirtyEight rated Biden’s chances of winning a delegate majority at 99%.

A stunning turnaround

It’s already hard to remember that in late February, it seemed entirely possible that Mr. Biden would drop out of the race around Super Tuesday. His turnaround was so quick that he gained about 36 points in national polls in 14 days.

“The polling swing toward Biden is probably the fastest in the history of the primaries,” tweeted FiveThirtyEight founder Nate Silver on March 9.

That swing was the result of changes in Democratic primary voters’ decisions. Given a one-on-one choice (the third remaining candidate, Rep. Tulsi Gabbard, has attracted little support) as a whole they clearly prefer Mr. Biden over Mr. Sanders.

But many important Democratic Party figures helped those voters along by winnowing the field and signaling through endorsements and other methods that Mr. Biden was now their own clear choice.

That means that in 2020, for the Democratic field, political experts who hold that the party writ large retains a disproportionate influence on nomination choices might be right.

“It is certainly evidence in their favor. But I believe there was a lot of contingency. I don’t think it was inevitable,” says Matt Grossmann, an associate professor of political science at Michigan State University.

In the 2008 book “The Party Decides,” Dr. Noel and co-authors argued that elements of organized political parties, from officials to special interest groups, can and do organize to support their chosen nominees in the “invisible primary” period before voting actually starts.

The 2016 election produced evidence both for and against their theory. On one hand, Hillary Clinton did indeed attract much institutional party support and money prior to the Iowa caucuses, making her the establishment choice. On the other, the Republican nomination went to a candidate the party hierarchy resisted, but seemed too weak and disorganized to stop: Donald Trump.

Then the 2020 cycle began with Democrats in the same position of the GOP four years before. Candidates were numerous, party coordination seemed weak, and a consensus establishment pick did not emerge prior to Iowa. Suddenly Sen. Sanders seemed on the verge of taking over a party which, while it has drifted left in recent years, organizationally remains resistant to Sanders-style revolution.

Big endorsement from Clyburn

“One question is why it took the party so long to get their coordination together, and if that delay means they really didn’t do anything,” says Dr. Noel in an email.

Maybe it was moderate Democratic voters in general, and African American voters in South Carolina and other Super Tuesday states, who lie behind the Biden resurgence, he says.

But Dr. Noel thinks the party did have an effect. South Carolina Democratic Rep. Jim Clyburn’s endorsement of Biden prior to the South Carolina primary was huge. Representative Clyburn is renowned in his state and the highest ranking African American in Congress. The party quickly coalesced around Biden after South Carolina, sending the same sort of signal that it usually sends prior to Iowa regarding the party’s choice.

This didn’t happen earlier because a lot of people in the party wanted someone else – Sen. Cory Booker of New Jersey, say, or Sen. Kamala Harris of California. It took time for these Biden alternatives to falter. Then came the flood: everyone from Mr. Buttigieg to Mr. Bloomberg ended their bids quickly, and moved to support the sudden front-runner.

Look at things from the point of view of the party, not the ambitious candidates, says Dr. Noel.

“From that point of view, it’s clear they were not comfortable with Sanders, but they weren’t sure how best to stop him, and not sure whether Biden could really win. The South Carolina results were enough to convince them,” he says.

Consternation in Sanders’ camp

To many Sanders supporters the sudden turnaround in Biden’s fortunes seems suspicious. All those people dropping out and endorsing Biden all at once? The Republicans couldn’t manage that in 2016. Somebody on the Democratic side must have made some phone calls.

President Trump, for one, has been eager to stoke this suspicion in an effort to split his opponents.

“I think it’s rigged against Bernie,” President Trump said in an appearance after Super Tuesday.

But on the other side, there are political experts who think the nomination system has too little input from party hierarchies and elected officials, not too much.

“Yes, I do believe that people who govern with the president of the United States and run on the same ticket have the most serious stakes in the nomination process. They should have a greater role,” says Elaine Kamarck, a senior fellow in the Governance Studies program at the Brookings Institution and author of “Primary Politics,” a history of the U.S. nomination system.

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Ms. Kamarck – herself a Democratic “superdelegate” with a vote at the convention if the nomination goes beyond one ballot – says a group of senators, representatives, governors, national committee members, and other high party officials should give a collective vote of “confidence” or “no confidence” for each candidate who wants to run for president.

“What’s happened in the 21st century is there are a lot of people in both parties who have no earthly reason to run for president, but do,” she says.