One of the major reasons that the political window in the United States has slowly shifted to the right for the last 60+ years is because the country lacked an organized and coherent political philosophy on the left. There's been environmentalist groups, feminist groups, LBGT groups, labor groups, etc, but no left-wing ideology that encompasses all of those values.
Those days are coming to an end.

DSA is now the biggest American socialist organization since WWII -- up from 8k members to 25k members in a year. pic.twitter.com/nTqeqABqEc

— Dave Weigel (@daveweigel) August 1, 2017

Nearly 1,000 socialists from around the country have gathered in Chicago for the Democratic Socialists of America’s biannual conference....Most of the attendees seem to be under the age of 35 with a smaller but large contingent of old-timers who’ve been with the organization for many years. There are young, scruffy, and often bearded men aplenty who fit the BernieBro stereotype. But according to the DSA’s figures on delegates in attendance, 40 percent of the Chicago crowd are women, and 20 percent are people of color.

The massive influx of young people makes this revival so relevant. This trend is reflected in American society in general.

The average age of DSA members has since 2015 dropped from 64 to about 30, according to an organizer. A May 2016 Gallup poll, conducted after most of the Democratic primaries, found just that 35 percent of Americans viewed socialism favorably. Among voters under 30, that number rose to 55 percent.

As you might imagine, two groups feel threatened by this trend: neoliberal Democrats and the entire right-wing.
For conservatives, this is like a zombie nightmare.

In a McCarthyite screed that appeared in the pages of National Review last March, the CEO of the Christian Legal Society, David Nammo, ominously warned the conservative publication’s readership about the growing popularity of socialism, and how this apparent shift in opinion is “threatening America’s future.”
...“Many Americans,” continues Nammo with a melodramatic intensity, “have forgotten the lessons of the Cold War and the disasters witnessed in the crumbling economies and failed polities of Communist and socialist countries in the 1990s … Americans who believe in limited government, welfare reform, and states’ rights should look over their shoulder and realize that a dangerous ideology is gaining ground.”

Comparing democratic socialism to totalitarian communism is laughable. Yet, it worked during the Cold War, in the same way that conservatives believed smoking pot was equal to shooting heroin during the Drug War.
The expiration dates of both fallacies have long since expired.

Alas, intellectual honesty is not typically associated with anti-socialist diatribes found in a magazine whose senior editor is the author of a book titled “Liberal Fascism.”
Nevertheless, while Nammo’s theory of why socialism is on the rise may be way off the mark, there is no doubt that socialism is on the rise in America — and that staunch conservatives like him have every right to be concerned.
...A conservative like Nammo would obviously take this to mean that our colleges and universities are indoctrinating the impressionable youth with socialist dogma, but there is a much more sensible and persuasive explanation for this socialist appeal. It goes something like this: Millennials are drawn to socialism because they have been royally screwed by the capitalist economy and the corporate state.

"From each according to their ability, to each according to their needs. Please donate to ensure that everyone who needs a drink gets one."
- sign at DSA Conveniton

Yet it is neoliberal Democrats that have the most to fear because they are much closer to the source.
The DSA is not an actual political party. It's more of an independent coalition, loosely tied to the Democratic Party.
Because the Democratic leadership is still dominated by neoliberals, means this rising Socialist movement is on a collision course with the mainstream political leadership.

It is tempting for liberals to see Sanders, DSA and their ilk as the enemy, rather than the vanguard, in the ongoing trials over the future of American liberalism. Yet the alternative is to fight for the status quo: endorse and line up behind other neoliberal Democrats like Sen. Cory Booker, N.J., Sen Kamala Harris, Calif., defeated Georgia congressional candidate John Ossoff, and perhaps even Facebook CEO Mark Zuckerberg. Yet this impulse will surely lead to ruin. Indeed, the Democrats lost their way in 2016 by capitulating to Clinton’s stale, empty vision of the future. Indeed, in a July 2017 poll, 52 percent of voters polled said that the Democratic Party stood for nothing aside from being “anti-Trump.”

The advantage of a rising socialist movement is that there exists a comprehensive and coherent ideology, combined with an energetic, organized political movement.
To give you an idea of what this looks like, consider the religious right and libertarians/anarcho-capitalists that have dominated political thought for generations. They weren't a majority, but they seemed like it because they were organized.
So what does the DSA believe?

“Under capitalism we are supposed to take for granted that a small, largely unaccountable group of corporate executives should make all fundamental decisions about the management of a company comprised of thousands of people,” Resistance Rising reads. “This group has the power to determine how most of us spend the lion’s share of our waking hours, as well as the right to fire anyone for basically any reason, no matter how arbitrary. Under democratic socialism, this authoritarian system would be replaced with economic democracy.”
That means, literally, the democratic governance of all firms by employees and local communities and the subjection of certain large industries—such as utilities, health care, and housing—to government planning and decommodification. Firms in most other industries, while “worker-owned and -operated,” would remain subject to market forces.