Democratic Representative Jake Auchincloss won his party's primary in Massachusetts' 4th Congressional District on Tuesday, seeing off his first Democratic challenger since taking the seat in 2021. He was contested from the left by Jason Poulos, an AI policy researcher and former Bernie Sanders campaign volunteer, whose defeat suggests limited appetite among the district's Democratic voters for a more insurgent, progressive alternative despite national tensions within the party over Israel policy.

Poulos campaigned on Medicare-for-all, universal basic income, taxing corporate automation, stronger unions and student-debt cancellation, alongside cutting military spending and ending US military aid to Israel, criticising Auchincloss's past support for such aid and his ties to AIPAC. Auchincloss, who is AIPAC-endorsed but has said some Israeli actions in Gaza in 2025 "crossed lines," had in July joined more than 100 House Democrats voting to strip $3.3 billion in military financing for Israel while still backing the broader US-Israel alliance; he ran instead on lowering housing and healthcare costs and preparing Democrats for AI-driven economic disruption. A former registered Republican who once worked for Governor Charlie Baker before rejoining the Democrats, Auchincloss succeeded Joe Kennedy III and comes from a prominent Boston medical family.