Why it matters: Rep. Diana DeGette (D-Colo.) was a staunch progressive, not a moderate, these members are privately fuming. So why did she become a target of the left?
"One more case in the growing dynamic of performative politics," one House Democrat, speaking on the condition of anonymity to share candid analysis on the results, told Axios.
"Diana was an excellent representative with seniority — but the style of someone younger and more outspoken has become more attractive to that cohort of motivated urban left voters."
A senior House Democrat called the result a "wake-up call" for members of Congress
Driving the news: DeGette was defeated decisively by 29-year-old Melat Kiros, an attorney and PhD student who led the incumbent by nearly 10 percentage points as of Wednesday morning.
DeGette's loss in the Denver-based district came despite a deluge of outside spending in her favor from groups tied to the Democratic establishment and AIPAC.
Progressive groups such as the Justice Democrats spent substantially in favor of Kiros as well, but their expenditures were greatly eclipsed by those of pro-DeGette outside groups.
Between the lines: DeGette boosters sought to defend her progressive bona fides, touting her support for Medicare for All, her opposition to ICE and her time as a Trump impeachment manager.
Pro-Kiros ads took aim at the 68-year-old incumbent's support from corporate PACs and votes in favor of Israel.
DeGette's loss comes after challengers backed by New York Mayor Zohran Mamdani unseated Reps. Dan Goldman (D-N.Y.) and Adriano Espaillat (D-N.Y.) last week, which already had House Democrats on edge.
What we're hearing: "Diana is a progressive. Sad to see her loss," a third House Democrat told Axios, adding that there is clearly "an appetite for newer, younger blood in some parts of the country.
"I told everyone that would listen in 2024, that Trump winning was like manna from heaven to DSA," said a fourth.
"That DSA is ... winning some safe seat primaries with these young white college educated voters is just no shock. It's literally their entire playbook."
The other side: "If DeGette didn't deserve a primary, Denverites wouldn't have elected Melat by 10 points," said Justice Democrats spokesperson Usamah Andrabi.
"All this outrage proves is the Democratic establishment's contempt for their own voters," he added.
"It's time for the Party to catch up to its own base who are no longer interested in politicians who call themselves progressive but are bankrolled by corporate lobbies and AIPAC while voting to fund genocide & ICE."
The intrigue: Some House Democrats aren't waiting too long to bring Kiros into the fold, with some even exulting in her win.
"I'm grateful for all Diana has done ... as our region's dean," Rep. Emily Randall (D-Wash.) told Axios. "I don't know Melat yet, but I sent her a note of congratulations and look forward to welcoming her to the team."
Rep. Ro Khanna (D-Calif.), who backed Kiros, told Axios: "The progressive movement is where the energy of our party is across the nation."
What's next: The left isn't done yet. They're pinning their hopes on a slew of progressive candidates to knock off as many as half a dozen more Democratic incumbents this cycle.