That the Supreme Court should announce its decision in Janus v. AFSCME Council 31 on the day after the stunning upset in New York’s 14th Congressional District is an accident of history, but it’s a noteworthy one. June 27, 2018, may well mark the day that two lines crossed in Democratic politics. One line, heading downward, represents the institutional power of the labor movement. The other, rising more slowly, represents grass-roots, liberal engagement.

The Janus decision was not as surprising as Alexandria Ocasio-Cortez’s Democratic primary win. It was widely expected that conservatives on the court would throw out the ability of public-sector unions to collect fees from represented employees who weren’t union members; a case similar to Janus resulted in a split decision in 2016 after the death of Antonin Scalia. But the ramifications of the decision are significant.

On the most superficial level, it may be hard to see why. Most of those represented by unions in the public sector are dues-paying members, according to data from the Bureau of Labor Statistics. The decision in Janus affects only those who are covered under employee contracts but who aren’t members. In many states they paid what are known as “agency” or “fair-share” fees to help unions cover the cost of negotiations. As of this decision, those fees are prohibited.

The obvious immediate effect will be to reduce the amount of money coming into the union. But unions anticipate a ripple outward: Fewer resources means less organizing. Less organizing means not only a limited ability to expand membership but also, in the face of constant employer pressure, a threat to the existing membership pool.

Over the past few decades, the density of union membership nationally has fallen significantly. The overall number of people represented by unions hasn’t changed that much, but the size of the labor pool has grown a lot, meaning that the percent of employees represented by a union has fallen from more than 20 percent in 1985 to 12 percent in 2017.

Much of that shift, though, has been a function of declining private-sector union membership. In 1983, 12 million people were members of private-sector unions in the United States. Last year, the figure was 7.6 million. In the public sector, meanwhile, membership grew over the past three decades. In 1983, less than a third of union members in the country were in the public sector. In recent years, more than half have been.

Since 2000, public-sector representation has held fairly steady, with about 9 percent of those represented in 2017 not being members of the union.

Put another way: Public-sector unions have been an anchor for the labor movement even as the power of the movement has waned. The Janus decision doesn’t simply target a part of organized labor —it targets a part of organized labor that’s become probably the most important part of the movement.

Here’s where we get to the politics. Organized labor has, for decades, been a critical component of Democratic Party politics nationally. Not every union supports Democratic candidates, just as every union member doesn’t vote for the party. But unions and union members have been an important part of turning out voters on Election Day and a key force pushing for economic policies benefiting working-class Americans. There are a lot of asterisks that float around those sentences, certainly, but it’s broadly the case that much of the institutional power of blue-collar America resided in the labor movement.

That term “institutional power” is important. Americans are certainly aware of the existence of the Democratic and Republican parties but don’t generally think of them as repositories for power. That’s a central function of a political party, though — to build and maintain power that can be used to elect candidates and pass legislation. Throughout history, that effort to build power in the two parties has been remarkably successful.

In 2016, though, it worked against them. Bernie Sanders’s campaign for the presidency that year was more successful than expected, in part because the Vermont independent leveraged the institutional power of the Democratic Party to get on ballots and participate in national debates. Had he run as a third-party candidate, that success would have been more limited. On the Republican side, Donald Trump’s co-opting of the Republican Party’s institutional power was even more successful. A decade ago he was a Democrat. Now he’s the leader of the GOP.

The labor movement has similarly worked to build institutional power that wasn’t tied to individuals or particular moments. But its institutional power was also co-opted, including, at some points in its history, by organized crime. At the same time, labor’s opponents in the business community and Republican Party have relentlessly sought to undermine its power, for obvious reasons. It has been struggling to rebuild what was lost decades ago while leveraging what power it still has to effect policy changes nationally and locally.

Complicating that effort is that institutions broadly have lost public confidence. More Americans are identifying as political independents than as Democrats or Republicans, itself in part a reaction to how parties have deployed their institutional power. In New York’s 14th District, Ocasio-Cortez, like Sanders and Trump before her, ran against the party as an institution explicitly — and successfully.

The rise of far-left policies within the Democratic Party is, to some extent, an effort by the party to absorb that energy into itself as an institution. But that external pressure coming from people such as Sanders and movements such as the Democratic Socialists of America is certainly also a function of the waning power of the labor movement. Pushing for better wages and employment conditions was a central part of labor’s history; its waning power and waning ability to push the Democratic Party on those issues opened up a space that Sanders and the DSA have worked to fill.

They’re also trying to build new institutions that can hold power. Sanders formed a group called Our Revolution that aims to get people elected on Sanders’s economic issues — the essence of building institutional power. The DSA is likewise working to elect its candidates and push its issues, strengthening its power base. So far, though, much of the success from the effort to push politics to the left has been a function of individuals — problematic for the long-term health of a movement. Building institutional power is hard, slow and subject to abuse. That’s why it’s better to maintain institutional power than to try to build it anew. Maintaining institutional power is, in fact, the entire point.

Which is again why June 27, 2018, is such a significant day. The Janus decision deals a significant blow to one of the last traditional worker-centered institutions in American politics, just as new institutions — or, at least, people — show electoral strength. Whether that new power is institutionalized over the long term remains to be seen.