A generational battle is taking place in US politics that could have profound consequences not just for the global left but also for the future of the planet. Ever since the freshman class of Democrats entered Congress in January, many of them young women of colour and supported by a young activist base, they have met resistance from more established members of the party. When the newly elected representative Rashida Tlaib, from Detroit, called Donald Trump a “motherfucker”, hours after being sworn in, she was the subject of finger-wagging from politicians and pundits. “I don’t really like that kind of language,” house judiciary chairman Jerry Nadler said. CNN’s Chris Cillizza wrote that Tlaib’s choice of words was a gift to the president.

When 29-year-old Alexandria Ocasio-Cortez surprised everyone by seeing off an established Democratic rival and going on to win a seat in Congress, her triumph – after a shock primary victory in New York over the longstanding incumbent, Joe Crowley, who was the third most powerful House Democrat – was played down by the House of Representatives Speaker, Nancy Pelosi, who chose to emphasise that voters had merely “made a choice in one district”. Since then, Ocasio-Cortez, who has become an internationally recognised figure, has faced a backlash from an array of party insiders – some of whom seem to resent her public profile and Twitter-star status – for her refusal to play by the normal rules of Washington politics.

Play Video 0:57 'A better world is possible': Alexandria Ocasio-Cortez elected to Congress – video

One member of the Democratic caucus told Politico: “She doesn’t understand how the place works yet”, while another suggested, somewhat condescendingly: “There’s a difference between being an activist and a lawmaker in Congress.”

The message is clear: the new, young politicians – and the activist movements that put them in office – need to sit down and learn how things work in the grownup world of Congress. But the new politics is conquering the old. However uncomfortable the style and tactics of this radical crop of young Democrats make the party’s elite feel, the truth is that young activists are successfully remaking the Democratic party from the bottom up.

Take, for example, the Justice Democrats, a leftwing grouping who are backing radical, diverse challengers to more established Democrats. Ocasio-Cortez’s New York triumph demonstrated their growing influence; and putting an unapologetically leftwing, working-class candidate in office is a sign of how times are changing. The issues Ocasio-Cortez and others like her are raising – the abolition of the US Immigration and Customs Enforcement agency, Medicare for All, a federal jobs guarantee – are starting to shift the party’s goalposts.

Nowhere is the influence of young activists more evident than in climate change. Days after the Democrats reclaimed the House, in the November midterms, activists from the Sunrise Movement occupied Pelosi’s office, forcing into the spotlight their demand for a Green New Deal – a radical plan to convert the country to renewable energy over 10 years. In February, it was the turn of the 85-year-old senator Dianne Feinstein to be confronted, again underlining the stark contrast in urgency between the generations. Bolstered by a new UN study that showed that the world has until just 2030 to head off climate catastrophe, Sunrise activists demanded politicians address the issue with the gravity that it deserves.

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In Congress, Ocasio-Cortez has helped roll out the Green New Deal resolution, which sets goals that include transitioning to renewable energy by 2030, guaranteeing green jobs and overhauling the country’s transportation and buildings. So far 11 senators – among them the presidential candidates Kamala Harris, Cory Booker, Bernie Sanders, Elizabeth Warren and Kirsten Gillibrand – and more than 80 House members have become co-sponsors of the resolution. This is a dramatic ideological shift, and one that can be traced directly to young activists who have refused to settle for incremental change. As one Sunrise Movement board member told the New Republic magazine: “Young people are the most politically liberated force in our country right now. We have less to lose than any other generation, and everything to gain. We can be radical. We can be visionary.”

The renewal of the US left, driven by youthful energy and activism, has been coming. Over the past decade, young people from movements such as Occupy and Black Lives Matter have drastically shifted the way we think about inequality and race. The latter is one of the most successful campaigns in recent history. For example, police brutality and mass incarceration have become such integral issues to the Democratic party platform – in large part thanks to Black Lives Matter activists – that having a career as a prosecutor has become a liability, rather than a strength, for 2020 candidates like Harris.

Young activists, and the politicians they have helped put in office, have blown open the ways in which politics is supposed to work. The question now is not whether or not they have succeeded in remaking the Democratic party but rather how fast and how far they will be able to go. All our futures depend on that answer.

• Clio Chang is a freelance journalist based in New York City