With President Trump’s second term in full swing and the midterm election four months away, a new kind of Democratic candidate is ascendant: the outsider vowing to overhaul the system.

The rise of these candidates shows a mood shift, many in the party say, from the midterms in Mr. Trump’s first term. Eight years ago, Democrats won the House after elevating scores of candidates calling for good government and incremental health care improvements. This time, many Democratic voters are making clear their hunger for systemic change — charting a course in the first half of the primary season that could point toward a different type of general election in the fall.

The party, which has often appeared rudderless since Mr. Trump’s return, is testing in real time whether it can go further in key races by running populist progressives who energize the base, or by choosing more traditional, centrist candidates.

In a marquee Senate race, Democrats picked a fiery, progressive Maine oysterman with scant political experience over a two-term governor. They sided with democratic socialists over House incumbents in deep-blue districts in New York and Colorado. They rejected the preferred candidates of the House Democratic campaign arm in battleground districts more than once. And they recently chose a state attorney general running as an outsider over a sitting senator in the race for Colorado governor.

The insurgents, often campaigning on progressive platforms, have injected new energy into a party still recovering from a crushing defeat in 2024. But they have also concerned some in the party establishment who worry that voters are choosing candidates with narrower general-election appeal or politically toxic positions that could hurt Democrats in November. Mr. Trump has stepped up his efforts to portray Democrats as extremists over the past week, falsely describing recent primary winners as communists.

In many other races, more traditional Democrats, including some familiar faces, have still found success. The centrist Josh Turek, the party’s Senate nominee in Iowa, beat a progressive primary rival. And the party is pinning its Senate hopes in Ohio on Sherrod Brown, a former senator, and in North Carolina on former Gov. Roy Cooper. The House Democratic campaign arm has gotten its preferred nominee in a range of targeted swing districts.

Still, the scale of the insurgent victories this cycle reflects the thirst for change from an angrier Democratic electorate, strategists and officials across the Democratic Party’s ideological spectrum say, and could have implications for November and beyond. Primaries later this summer could show how far the disruptive, populist approach is spreading.

“People are starting to realize that Donald Trump was not actually a mistake,” said Dr. Abdul El-Sayed, a progressive running for Senate in Michigan. “He reflected a real frustration in the electorate that Democrats have routinely failed to really address.”

Dr. El-Sayed fell far short in a run for governor in 2018, losing in the primary to Gretchen Whitmer, now the governor. But he leads, according to recent polls ahead of the Aug. 4 primary, over Representative Haley Stevens, a moderate running with support from party leadership. Mallory McMorrow, a third candidate who tried to bridge the divide between Dr. El-Sayed and Ms. Stevens, suspended her campaign on Sunday without making an endorsement.

Many of the Democratic upstarts who have gained traction this year have been critical of Israel, embraced universal health care, run on generational change and advocated a more combative posture against Mr. Trump.

“It’s not enough for our position to be that we’re simply going to return to a pre-Trump status quo,” said Senator Chris Van Hollen, Democrat of Maryland, who was chair of Senate Democrats’ campaign arm in 2018. He has endorsed Dr. El-Sayed and other rising progressives this cycle.

In May, a New York Times/Siena poll found that more than half of Democrats and Democratic-leaning independents were frustrated with the party. And more than 80 percent of the party’s supporters said the political and economic system needed to be torn down entirely or overhauled in a major way.

But the party is fractured over who should lead the rebuilding — and how far-reaching it should be.

Graham Platner, who won the nomination for Senate in Maine despite unsettling disclosures about his past, and Gov. Janet Mills, whom he handily defeated, haven’t spoken since his primary victory, he said last week.

In purple Wisconsin, Francesca Hong, a democratic socialist, is seeking the nomination for governor, a prospect that has alarmed some Democrats.

In New York, three self-described democratic socialists won House primaries in deep-blue districts last month. Their victories echoed the 2018 win of another insurgent progressive in the state: Representative Alexandria Ocasio-Cortez, who upset a high-ranking House Democrat. (Ms. Ocasio-Cortez rose to the House with other progressives, including Rashida Tlaib of Michigan; Ilhan Omar of Minnesota; and Ayanna Pressley of Massachusetts.)

One of those victors in New York was a political newcomer, Darializa Avila Chevalier, who had for years posted on social media in crass and profane terms, at one point using an expletive to attack Vice President Kamala Harris. (Ms. Avila Chevalier has said she regrets the way she expressed herself in her old posts, which were published on an account she later deleted.)

Two days after the New York primary, a group of moderate Democrats signed a letter pushing back on the philosophy of the ascendant Democrats. “We are capitalist, not socialist,” said the letter, whose signatories worry that Republicans will find success casting the entire Democratic Party as outside the mainstream.

Representative Tom Suozzi, a centrist from a swing New York district who helped lead the letter, said the rise of democratic socialist candidates was an understandable response to politicians’ “tone-deafness” in the face of an affordability crisis. But, he said, Democrats were “never going to win the majority by just appealing to the extreme base.”

“The Republicans are always going to get 40 percent,” Mr. Suozzi said. “The Democrats are always going to get 40 percent. It’s the fight over that 20 percent in the middle.”

The contours of the general election are not yet clear, and Democrats expect to once again capitalize on Mr. Trump’s unpopularity. They accomplished that in 2018 with fall campaigns designed not to alienate suburban swing voters.

But Jon Fleischman, a Republican strategist, said that Democrats were “squandering a golden opportunity” this cycle to win voters in the middle against far-right nominees from his party.

Mr. Fleischman said that Democrats would be positioned for “landslide wins” in key campaigns if they chose more moderate candidates, but were instead headed for “very competitive races.”

“This isn’t complicated,” he said. “If you occupy the center to the left, you’re going to win. That’s a huge coalition.”

Matt Dunlap, running as a progressive, captured the nomination in a conservative-leaning House district in Maine after arguing that the district’s departing Democratic congressman, Representative Jared Golden, had sided with Republicans too frequently. (Mr. Golden won the seat in 2018.) The House Democratic campaign arm preferred a more centrist candidate, but some Maine Democrats say that party leaders in Washington, D.C., underestimate Mr. Dunlap’s strength.

While some see primary wins by progressive insurgents as detrimental to Democrats’ chances in November, others argue that the firebrands will generate enthusiasm that has been lacking in the party.

Some of the insurgent Democrats who have scored victories in primaries, especially in deep-blue districts, are not all that different ideologically from the establishment Democrats they defeated. But certain issues, including the war in Gaza, have deeply divided Democrats.

Senator Kirsten E. Gillibrand of New York, chair of Senate Democrats’ campaign arm this cycle, acknowledged that it would “take time” for her party to come together after a series of divisive primaries, but she said she was confident that the party would.

The country, she argued, is undergoing a “massive shift” that has far more in common with the 2006 midterms of President George W. Bush’s second term — when Democrats rode anger over the Iraq war to control the House and Senate — than with the 2018 midterms, when Democrats took the House but not the Senate.

“People will vote differently than they ever have before,” Ms. Gillibrand said. “Because they’re not happy.”

On election night in 2018, Representative Nancy Pelosi, who would soon return as speaker, said the election was in part about “restoring the Constitution’s checks and balances to the Trump administration.” Two years later, the party rallied around Joseph R. Biden Jr., who vowed to “restore the soul” of the country as president. For some Democrats, those days are now over.

“In 2018 it used to be about restoration,” Jesse Ferguson, a Democratic strategist, said. “Now it’s about renovation.”