WILMINGTON, Del. — Sen. Thomas R. Carper (D-Del.) held off a challenger to his left in Thursday’s primary, decisively defeating activist Kerri Harris in one of the last intraparty brawls of the year.

Carper, 71, is now favored to win a fourth term in a state where his center-left politics have helped shape its business-friendly Democratic Party. But the challenge from Harris, a 38-year old Air Force veteran making her first run for office, revealed a shift in Carper’s party and the existence of a left-wing vote bloc in a state where moderate suburbanites typically dominate.

That bloc was not large enough to give Carper a scare, however: Harris got 35 percent of the vote, easily eclipsed by Carper’s 65 percent.

The senator, who has won 12 general elections for various offices in Delaware — and lost none — did not immediately seem vulnerable to a challenge. Republicans, who were also picking a nominee Thursday, struggled to find a first-tier challenger; Sussex County councilman Rob Arlett claimed the nomination Thursday with 67 percent of the vote.

In the first six months of her campaign, Harris raised less than $25,000, not enough to run a credible race in a state with 367,000 registered Democrats.

But the June 26 primary victory of New York’s Alexandria Ocasio-Cortez was a booster shot for the left, and for insurgent groups such as Justice Democrats and the Working Families Party. Both made their first-ever interventions in Delaware politics, as Ocasio-Cortez’s campaign sent staff to Delaware to build a voter-targeting campaign for Harris.

“Fool me once, shame on me,” said Carper, who has said he will oppose Kavanaugh’s current nomination.

No Democratic senator has lost a primary since 2006, when then-Sen. Joseph I. Lieberman of Connecticut responded to his defeat by mounting and winning an independent campaign. But the idea of building a liberal beachhead in Delaware, and of signaling to other Democrats that they could not take activists’ votes for granted, proved attractive to insurgents.

Some of Ocasio-Cortez’s key campaign staff headed to Delaware in July. After the Aug. 7 Michigan primary, when liberal gubernatorial candidate Abdul El-Sayed fell short, his digital director, Claire Sandberg, headed east to direct a $100,000 independent expenditure on Harris’s behalf, paid for by the New York-based Working Families Party.

The primary election that loomed over Delaware Democrats happened in 2010. That was the year that longtime House member Mike Castle, a moderate Republican running for a then-open Senate seat, was defeated by conservative activist Christine O’Donnell. The scandal-plagued O’Donnell went on to lose to now-Sen. Christopher A. Coons (D), bucking a national trend that year in favor of Republicans.

Harris and her supporters insisted that she would hold the seat for the Democrats in November, pointing to Delaware’s blue tilt — Donald Trump won just 41.7 percent of the state’s vote — and Republican polling that has found her narrowly ahead of any of their candidates. The chance to elect the first black female gay senator of any state, Democratic activists said, would also have attracted donors and activists who had otherwise ignored the race or gotten behind Carper.

“The DNC would say, ‘Hey, we’re worried,’ ” Harris admitted. “But the wonderful thing about Delaware is that we’re small. We talk to each other. And we have a formula to get people to the polls.”

Republicans, who once hoped that Carper would retire this year, had a contentious primary of their own, between former PayPal executive Gene Truono and Arlett, who chaired Trump’s campaign in the state.

The GOP’s race didn’t received the same attention — the candidates spent less than $300,000 combined on the primary — but veered into nastiness. Asked by the state’s biggest newspaper, the News Journal, what the biggest difference between their campaigns was, Arlett said, “I’m married to a woman, and he is not.”

Truono is openly gay — and won the paper’s endorsement. But he received only 28 percent of the vote Thursday.

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