The news broke late Wednesday night in The Washington Post:

The Justice Department has granted immunity to a former State Department staffer, who worked on Hillary Clinton’s private email server, as part of a criminal investigation into the possible mishandling of classified information, according to a senior law enforcement official.
The official said the FBI had secured the cooperation of Bryan Pagliano, who worked on Clinton’s 2008 presidential campaign before setting up the server in her New York home in 2009.

That latest development in the ongoing FBI investigation of Clinton's private server came on the same night Clinton campaign manager Robby Mook declared in a memo that it would soon to be "mathematically impossible" for Bernie Sanders to catch up to Clinton for the Democratic presidential nomination, given her current delegate dominance.

So …

Here's the thing: The Democratic establishment, a long while back, put all of its eggs in the Clinton basket. No one of considerable stature in the party challenged her for the nomination (Elizabeth Warren and Joe Biden, I am looking at you), and virtually every member of the party's power structure got on board with her very early on.

That bandwagoning around Clinton has been overshadowed over the past few months as Sanders's surprisingly strong challenge to Clinton (and his amazing small-dollar fundraising) has drawn much of the focus. But the truth of the matter was and is that the Democratic Party made a massive bet on Clinton about two years ago. That bet was that she was the strongest possible candidate they could field and so it only made sense to push everyone else to the side for her.

But, it was a bet. And, like all bets, there was -- and is -- risk involved. That risk was that somewhere before the 2016 general election something might happen that would make it more difficult -- or even impossible -- for Clinton to win. This is the Clintons we are talking about, after all. For all of their smarts and deep résumés, there does tend to be some serious baggage that trails them wherever they go.

No one -- or at least no one outside Clinton's inner circle -- would have known way back when that the "Hillary or bust" gamble was made that she had exclusively used a private email server while secretary of state and that a number of pieces of classified material had passed through that server. (There is no debate that classified information was on Clinton's server; the debate is whether it was classified at the time or whether it was classified at a later date.) That news didn't break until March 2015, when the lining up behind Clinton was already very much underway.

But, here we are. And, Mook is right. Clinton's delegate lead coupled with the fact that Democrats only allow proportional allocation in primaries and caucuses makes it very, very hard for Sanders to win. Her dominance on Super Tuesday confirmed for anyone who was paying attention that the nomination is Clinton's now. Sanders can run for as long as he wants -- and has the money to do so -- but it's virtually impossible to see how he catches or passes her in delegates.

All of which should make Democrats very nervous that Pagliano, the 2008 Clinton campaign staffer who set up the private server in 2009, has been granted immunity by the Justice Department. (Pagliano had previously invoked his Fifth Amendment rights against self incrimination despite the fact that Clinton's team insisted that he should testify about his role in setting up the server.)

The kindest possible reading of this news for Clinton is that Pagliano was simply nervous to talk about how -- and why -- he had set up the email server, and granting him immunity lets him speak freely without any concern that he might get into trouble.

Maybe. But it's my strong impression that the Justice Department doesn't go around granting immunity to people unless the person getting the immunity may be able to shed light on an important part of the investigation. After all, if Pagliano a) knew nothing or b) did nothing wrong, why would he need immunity to talk to the FBI?

That's the question skittish Democrats have to be asking themselves today. The granting of immunity to someone at the center of the email controversy will be taken as a sign that things may get worse for Clinton when it comes to the email server before they get better.

All of the party's chips are on Clinton. And that's where they will have to stay -- almost no matter what.

Hillary Clinton on the campaign trail

CLEVELAND, OH - On the third day of a bus tour through Pennsylvania and Ohio, Democratic Nominee for President of the United States former Secretary of State Hillary Clinton along with running mate Senator Tim Kaine, and Anne Holton, aboard the campaign bus in Cleveland, Ohio on Sunday July 31, 2016. (Melina Mara/The Washington Post)