When Ayanna Pressley topped a 10-term congressman in a Massachusetts district once represented by John Fitzgerald Kennedy, she became the latest face of a burgeoning movement of the grassroots left. "This is a fight for the soul of our party and the future of our democracy," Ms Pressley said. That movement is reshaping a Democratic Party still searching for leaders and identity in the era of Donald Trump.

What’s happening?

Grassroots activists on the left have a mixed record in Democratic primaries this election year, but the self-described progressive movement has scored enough victories to suggest its popularity is based on more than just protesting the Trump White House. The movement is also remoulding the Democratic Party into a younger, more diverse and decidedly liberal party.

Ayanna Pressley and New York's Alexandria Ocasio-Cortez, two women of colour, both defeated Democratic incumbent congressmen in their primaries. The progressive movement has helped nominate three black Democrats for governor's seats, including a clear upset in Florida, where Tallahassee Mayor Andrew Gillum knocked off establishment favourite Gwen Graham. Mr Graham's establishment credentials could not have been stronger. Her father, Bob, is a Florida icon who served as governor and US senator.

Georgia's Stacey Abrams, meanwhile, trounced a primary opponent recruited and backed by much of her state's Democratic old guard. Ms Abrams would become the first black woman elected governor in any US state.

Scores of other down-ballot candidates are running for local, state and federal offices with backing from grassroots groups like Indivisible, MoveOn.org, the Working Families Party and the offshoot of Bernie Sanders' 2016 presidential campaign, Our Revolution.

Why it matters

The diversifying, more liberal slate will be measured by the outcome in the November vote.

Republicans — and some Democrats — argue that the party is going too far to the left for American voters, particularly outside of the cities and close-in suburbs where Democrats' base of white liberals and non-white voters is concentrated.

Progressive nominees like Ms Pressley and Ms Ocasio-Cortez are running in Democratic strongholds, so their wins in November are all but guaranteed. But Republicans will use the rise of the left — and its support for policies like single-payer health care and scrapping US Immigration and Customs Enforcement — to tar all Democratic candidates.

The counter argument from Democrats and activists on the left is two-fold: 1) Many issues that more liberal candidates support — some gun regulation, expanding Medicaid, raising the minimum wage — actually have widespread public support, even among independents and some Republicans; and 2) the more openly liberal candidates from outside the old-guard establishment can bring in new voters who do not regularly cast midterm ballots.

Beyond November, the new generation of Democrats will have a say in how the party governs. Ms Pressley and Ms Ocasio-Cortez will not determine whether Democrats pick up the 23 new House seats they need for a majority, but they will help shape the arguments within the Democratic caucus.

They and their fellow freshmen will have a say in whether a new Democratic majority returns Nancy Pelosi to the speaker's chair. They will have leverage — much like the arch-conservative Freedom Caucus does with the House GOP leadership — over every debate, from health care to the potential impeachment of Mr Trump.

So, for example, while there might not be enough support for a real push toward single-payer health care, the movement could draw the party toward supporting a public health insurance option to compete alongside for-profit companies selling policies in Affordable Care Act exchanges. And, even if an immediate $15 minimum wage is too heavy a lift, perhaps the left-flank forces a compromise of graduated raises over time.

What to watch out for

Before primary season concludes, there are a few more key match-ups to measure the left's influence. In Delaware, military veteran and activist Kerri Evelyn Harris is aiming on Thursday to knock off moderate Democratic Senator Tom Carper. She is a big underdog, but even a competitive finish will serve notice anew that incumbents must at least contend with the left base. Similar dynamics exist in New York, where actress Cynthia Nixon is trying to topple Governor Andrew Cuomo in a 13 September primary.

Some of the most intent observers of the party's evolution are the gaggle of aspiring presidential candidates. Certainly, some of those figures have helped drive the shift — Mr Sanders and his 2016 campaign, Massachusetts Senator Elizabeth Warren and her outspoken economic populism.

But Mr Sanders, Ms Warren and the rest are also careful to watch the base and not get caught flat-footed as it evolves. That is one reason potential candidates like Senators Cory Booker and Kamala Harris and former Vice President Joe Biden have made sure that their endorsement and campaign finance activity this midterm cycle has spanned the spectrum of the party's down-ballot candidates: If the so-called progressive movement becomes the centre of the party, no one who wants to be the Democratic presidential nominee boxed out.