Representative April McClain Delaney of Maryland won a nail-biter of a Democratic House primary race on Tuesday, according to The Associated Press, holding off a fierce, multimillion-dollar challenge from a former representative who had pulled from his personal fortune in an aggressive bid to reclaim his old seat.

With her victory, Ms. McClain Delaney, a lawyer and former official in the Biden administration, will advance to the general election in Maryland’s Sixth District. She is seeking a second term in a solidly Democratic seat in western Maryland.

The race, one of the most expensive congressional primaries in recent history, was a bruising intraparty battle over ideological purity and immigration policy. By June, the two leading candidates had raised $10 million more than candidates in the most expensive House race in 2024, according to campaign finance filings— $35 million in total — with months left to go until the November general election. A majority of fund-raising came from loans the candidates made to their own campaigns.

Ms. McClain Delaney’s top opponent, former Representative David Trone, lent himself $25 million in a bid to unseat the very incumbent he had once backed as his successor. When Mr. Trone left his House seat for an unsuccessful 2024 run for the Senate, he endorsed Ms. McClain Delaney to fill the seat.

In his campaign, Mr. Trone focused heavily on Ms. McClain Delaney’s vote in January 2025 for the Laken Riley Act, a Republican-backed bill that requires federal law enforcement to detain undocumented immigrants who have been accused of certain crimes without bail.

Ms. McClain Delaney, who was the only Maryland Democrat to vote for the measure, has said that she regrets doing so. She claimed in her campaign ads that she had a record of voting “100 percent against Trump’s ICE,” though she had voted for the Laken Riley Act two days after President Trump took office.

The congresswoman “initially voted for (the act) out of a sincere desire to address community safety concerns following a tragedy,” Nick London, Ms. McClain Delaney’s campaign manager, said in a statement that accused Mr. Trone of distorting her record.

Ms. McClain Delaney benefited from a firewall of support from the Maryland Democratic establishment, securing endorsements from nearly every prominent Democrat in the state.

In the race’s final weeks, high-profile elected officials like Representative Jamie Raskin and Senator Chris Van Hollen appeared in her advertisements, defending her as “progressive and effective.” She was also buoyed by over $500,000 in support from Protect Progress, a pro-cryptocurrency super PAC, and she leaned into her own wealth, lending her campaign $7.4 million.

The district, historically a comfortable hold for Democrats, voted for former Vice President Kamala Harris by 10 percentage points in 2024. Ms. McClain Delaney’s family also has deep roots in the seat; her husband, John Delaney, represented the district for three terms before his failed presidential bid in 2020.