What divides America? This weekend, it was a Chicago street.

The unrest at a Trump event was a symbol of a nation of partisans who don't trust each other. But it also highlighted a way forward.

For one night, Chicago's Harrison Street might as well have been Capitol Hill. 

On one side of the street stood supporters of Donald Trump, upset that a campaign rally had been canceled. On the other stood protesters against Mr. Trump – the reason the rally had been canceled. By the end of the night, the tension led to violence and recriminations.

For all the grandiose talk this primary season about whom America should choose as president to "fix Washington" or "make America great again," it was right there – on that Friday night in Chicago – that America's democratic experiment was playing out on its most basic level.

The protests, violence, and general chaos that enveloped the presidential race this weekend gave the impression that American politics was spinning out of control. To the contrary, it was further evidence that the American political process has perhaps never been under tighter control. Not of the establishment. Of the voters.

Friday night in Chicago showed that there is little mystery about politics today. Washington is divided and angry because American voters are divided and angry. In the not-too-distant past, parties acted as a buffer, wringing some measure of concord from Congress's cacophony of voices. No more. 

The parable of this election so far has been the impotence of the establishment. Just think how many times and ways the Republican establishment has tried to get Trump to play by anything approaching the rule book. And who in the Democratic National Committee really wants Bernie Sanders to still be in the race?

The trend has been building for years. Former House Speaker John Boehner did not want the tea party agenda. The voters forced his hand. The parties are by no means dead, but their influence is a shadow of what it once was. 

Which brings America right back to Harrison Street. What this weekend showed, in the starkest terms yet, was that the only practical solution to the problems that seem to beset America is the American voter. 

If red and blue America – separated by a few feet of asphalt and wildly different worldviews – can't find a way to get along, then Congress doesn't have tools to, either. Nor would any president.

Despite claims to the contrary, the establishment can do little of substance without voter buy-in. In searching for the element that has unified Trump voters across economic and geographic groups, ABC News found that the single most predictive factor was anger at the establishment.

Compared with other factors, an ABC News poll found that "the idea that Trump’s popularity is fundamentally based on anger against the existing political establishment, and the sense that an outsider is needed to fix it, have significantly more legs."

Some 82 percent of Trump supporters say they prefer an outsider, the poll found. 

Indeed, America sits at a unique confluence of political history, suggests Bruce Schulman of Reuters. The influence of the parties is declining as the partisanship of voters is rising. The result is that tensions are increasing at a time when the parties' ability to manage them is diminishing.

"For much of the nation’s history, partisan attachments burned just as hot – if not hotter – than they do today, but strong party organizations disciplined their members and formed effective tools of governance," he writes. "As party organizations weakened and partisan ties gradually atrophied after World War Two, space opened up for the influence of a wide variety of interest groups and the emergence of different, but nonetheless workable models of policymaking. Now, fierce partisanship has reemerged – but without effective party organization or authority to police it."

The reasons for the decline of the party are many, he says, from the rise of organized interests to "mass media supplanting the party organization as the principal intermediary between elected officials and ordinary voters."

But the development leaves the country with no political referees. The task of governing falls largely to the voters by default. And at this moment, that is a recipe for division. 

More than a quarter of Democrats and Democratic-leaning independents (27 percent) said that the other party's positions “are so misguided that they threaten the nation's well-being,” according to a 2014 Pew Research Center study. More than one-third of Republicans and Republican-leaning independents (36 percent) said the same. These make up the core of primary voters.

As the Monitor's Peter Grier noted in a cover story, swing voters are disappearing. Even declared independents tend to vote for one side consistently, and many are voting against a party rather than for it. 

Parties have played their part in fueling antagonism for the other party. Voters have also sorted themselves along partisan lines in recent years – meaning there are few conservative Democrats or liberal Republicans anymore. 

The result is a political system that has little common ground. Weak parties "may well make effective governance all but impossible," Mr. Schulman of Reuters says.

But the decline of parties also makes the way forward apparent, if not easy. It comes down to what voters want, and the solution to the status quo is clearly not further polarization.

"There is a tendency on the left and the right to associate primarily with like-minded people, to the point of actively avoiding those who disagree," the Pew study found. "Not surprisingly, this tendency is also tightly entwined with the growing level of partisan antipathy. In both political parties, those with strongly negative views of the other side are more likely to be those who seek out compatible viewpoints."

This weekend, that divide was not blue-red or Republican-Democrat, but a Chicago street.