Sen. Bernie Sanders (I-Vt.) at a town hall event in Bethlehem, Pa., on Monday. (Matt Rourke/AP)
Opinions editor

Monday was supposed to be a rough day for Sen. Bernie Sanders (I-Vt.). First, the Vermont senator and candidate for the Democratic presidential nomination would finally release 10 years of tax returns after months of foot-dragging; many assumed the returns must contain something bad if their release was so delayed. And, that evening, he would take part in a Fox News town hall, against the wishes of many Democrats who wanted him to boycott Fox News or who feared the cable network would embarrass him.

But his tax returns turned out to be “boring” — just as he’d promised. The town hall was a ratings smash and spawned several viral moments, including when the audience overwhelmingly cheered for Medicare-for-all. The “avowed socialist,” as political journalists like to describe him, sailed through the toughest day of the campaign with flying colors. And he remains the strongest candidate in the polls behind former vice president Joe Biden.

No wonder the Democratic establishment is starting to worry.

“Many Democrats, especially donors and party officials," writes the New York Times’s Jonathan Martin, “. . . are growing more alarmed about Mr. Sanders’s candidacy.” He continues:

The matter of What To Do About Bernie and the larger imperative of party unity has, for example, hovered over a series of previously undisclosed Democratic dinners in New York and Washington organized by the longtime party financier Bernard Schwartz. The gatherings have included scores from the moderate or center-left wing of the party, including Speaker Nancy Pelosi of California; Senator Chuck Schumer of New York, the minority leader; former Gov. Terry McAuliffe of Virginia; Mayor Pete Buttigieg of South Bend, Ind., himself a presidential candidate; and the president of the Center for American Progress, Neera Tanden.

To be sure, all contested primaries see factions plotting to stop other factions’ candidate. But the sweep of the opposition named here is striking. The House speaker, the Senate minority leader, a former chairman of the Democratic National Committee, the head of the most powerful liberal think tank, and the new darling of center-left Democrats — that’s a murderer’s row of eminences. When Barack Obama jumped in against Hillary Clinton during the 2008 election, for example, he had key party figures such as then-Senate Majority Leader Harry M. Reid (Nev.) in his corner. Sanders has no such allies.

At the root of this is the power of the dollar. As The Post’s Dave Weigel pointed out, what scares Democrats most about a Sanders nomination is “the thing they can’t really say: Howard Schultz would charge in and hand the election to Trump.” Furthermore, while there’s little polling evidence to suggest Democratic voters would abandon the party if Sanders were the nominee — he polls as well or better against President Trump than any other contender aside from Biden — there’s plenty of reason to think that Democratic donors may do so. Martin reports on a gathering of “about 100 wealthy liberal donors in San Francisco last month” worried at the prospect of a Sanders nomination, for example, and it’s no coincidence that the series of “Stop Sanders" dinners reported in the Times’s story has been organized by a major donor to the party.

Beyond donor skepticism, there will also be attacks from business interests. On Tuesday, for example, UnitedHealthcare chief executive Steve Nelson pointedly attacked Sanders’s Medicare-for-all proposal, saying it would “destabilize the nation’s health system.” This follows The Post’s Jeff Stein’s report that the health insurance industry is lobbying Democrats intensely to reject more progressive health-care plans that “would effectively legislate many of the companies out of existence.” On this and other issues, a Sanders-led ticket would face more corporate spending against it in the general election than almost any Democratic nominee aside from Sen. Elizabeth Warren (D-Mass.).

So establishment Democrats are right that Sanders would face certain obstacles that most other potential nominees wouldn’t — namely, that more big paid-for megaphones will be turned against him. In the worst-case scenario, a billionaire would all but buy a campaign and likely hand Trump reelection. But rather than confront what that says about our political system, establishment Democrats are fanning the fears. They will tell you, of course, that they share the same goals: fixing health care, fighting inequality, taking on climate change, and so on. But as recent decades have showed, progress toward those goals will never be better than halting so long as moneyed interests are allowed to remain so powerful. Not actively fighting this broken system only helps perpetuate it.

Read more:

Karen Tumulty: Dear Bernie Sanders: Welcome to the 1 percent

Helaine Olen: How Fox News accidentally revealed the truth about support for Medicare-for-all

Greg Sargent: A Bernie Sanders exchange on Fox highlights a big Trump vulnerability

Dana Milbank: Bernie Sanders has emerged as the Donald Trump of the left