Hillary Clinton's first firewall against Bernie Sanders was going to be South Carolina, then the Super Tuesday states. With each Sanders victory somewhere else, it shifted further out, even as she built up an imposing lead in the Democratic nomination race.
Her latest chance to seal the deal is New York's primary next Tuesday and five more northeastern state contests the following week.
But Clinton's real backstop on her slow march to the Democratic nomination isn't a state. It's the buffer of more than 400 party officials and officeholders who've pledged their support to her as superdelegates to this year's Democratic National Convention.
Sanders and his aides have recently been floating a scenario for overtaking Clinton by collecting the relatively small number of superdelegates still unaligned and persuading some of her backers to switch. That may give hope to Sanders' most ardent voters and donors, but even some of the Vermont senator's own superdelegates don't give that much of a chance.
"I don't think I can make a good case for it," said Bert Marley, the Idaho Democratic Party chairman and an enthusiastic Sanders superdelegate, when asked how Sanders could win over Clinton superdelegates. "If you're a true believer you just hang in there and hope something materializes that makes it work; that's where I'm at at this time."
Superdelegates, a group comprised of Democratic U.S. lawmakers, governors, past presidents and national party officials who have the freedom to back their candidate of choice regardless of how their state voted make up 15 percent of 4,765 total convention delegates available. The rest are apportioned based on primary and caucus results, and 2,383 delegates are needed to claim the Democratic nomination.
The superdelegates overwhelmingly support Clinton so far: she has pledges from 469 and Sanders has 31, according to the latest Associated Press tally. There are a little more than 200 who are publicly uncommitted. She also leads Sanders in pledged delegates 1,289 to 1,038.
Sanders camp argues that Clinton's superdelegates in states he won should bow to the will of the voters and switch allegiance. They also are counting on the candidate making a big surge in the last stretch of contests.
"The one place where people may want to reconsider is states where Bernie had really huge victories,'' Tad Devine, chief strategist for the Sanders campaign said, citing Washington, Minnesota and Alaska among other states. "If we can really succeed heavily on the back end of this process, particularly in California" that would allow the campaign to make a strong case to the superdelegates that Sanders can win the general election.
"I think we've been pretty honest with ourselves and how difficult it would be," Devine said.
In the eight nomination campaigns since Democrats started using superdelegates, they've never upended the candidate with the lead in pledged delegates. Even in 2008, when Clinton was in a closely fought race against then-Sen. Barack Obama, only about 30 superdelegates abandoned her campaign before she released them once the race was settled. Clinton entered 2008 with a 3-to-1 lead over Obama in publicly committed superdelegates; she entered 2016 with about a 45-to-1 advantage over Sanders.
There's also a hard political calculation. Sanders is running against the political establishment, and the superdelegates are the very embodiment of that establishment.
"He wants to woo the people he has scorned," said former Pennsylvania governor Ed Rendell, a Clinton superdelegate. "He's insulted all of us and the process at the beginning. We are scorned lovers and it's not easy to woo us back."
In addition, some Sanders supporters have engaged in a very aggressive campaign to get Clinton superdelegates to switch, and Rendell said that's "creating a very strong backlash."
If Sanders "somehow, miraculously" won both the pledged delegate count and popular vote, Rendell said, he would probably have to reconsider his vote at the convention. But he predicted many Clinton superdelegates would not, even in that scenario.
"There are plenty of superdelegates who think Bernie Sanders has gotten a free ride" in the primary and would "implode" in a general election, Rendell said. "And, therefore, they'll stick with her."
Sanders supporter Larry Murakami, a first-time superdelegate who is vice chairman of the Alaska Democratic Party, said he still believes Sanders can switch some superdelegates if he overtakes Clinton in pledged delegates before the convention but that other scenarios are harder to imagine.
"If he can win the delegate count he can win," Murakami said. To that end, Murakami said New York's contest on Thursday is crucial. "It'd be best if he wins both New York and California."
While 2016 is an anti-establishment year, the sentiment has been far more pronounced in the Republican race, elevating billionaire reality TV star Donald Trump as the front-runner in a party nominating system that doesn't have superdelegates. Even still, Trump's campaign has stumbled in dealing with the arcane party rules awarding pledged delegates.
Clinton and her advisers have always said they'd compete for the pledged delegate lead and not have to rely on superdelegates to secure the nomination. She also remains ahead of Sanders by about 2.3 million in the popular vote count.
"I can't foresee any circumstances under which going into the convention I'd change my mind," said Randi Weingarten, a superdelegate who has committed to supporting Clinton this election, and who is president of the American Federation of Teachers, which has endorsed Clinton.
"Many of the people who support her have worked with Bernie in the Senate," Weingarten said. "There's 40 sitting U.S. senators who have endorsed Hillary. They see it up close and personal. I don't like the word firewall. What you're hearing from the political and party delegates is a significant amount of thought that went into supporting her, backed by lots of experience."
While some Sanders supporters have argued the influence of superdelegates is undemocratic, Weingarten pointed to the caucuses where Sanders has scored most of his victories "Somebody could argue that a caucus structure for two hours on one day is not terribly democratic," she said.