Immigration has become a U.S. political battleground where sovereignty, security, and electoral legitimacy collide on digital platforms rather than at the border itself. This article examines how partisan discourse on X (formerly Twitter) evolved from structured pre-2025 argumentation into acute communicative conflict following Trump’s return to the presidency. Treating executive actions as mediated events rather than evaluating policy, the study analyzes platform discourse as strategic public argumentation within an international media field. The analysis draws on three X datasets (2,847 raw posts; 1,243 filtered) collected during 2023–2024, establishing a baseline before Trump’s second-term executive orders. A mixed-methods design combines quantitative mapping of posting intensity, engagement, frame distribution, and argumentative strategy with qualitative critical discourse and pragma-dialectical argumentation analysis. Findings reveal that Republican discourse overwhelmingly securitized immigration through crisis, invasion, and governance-failure frames, while Democratic discourse attempted counter-securitization through bipartisan credentialism, administrative competence, and bounded humanitarianism. Biden’s June 2024 proclamation marked a turning point, conceding the security frame while seeking procedural legitimacy, yet Republicans reframed it as insufficient. Trump’s January 2025 executive orders completed the migration of platform rhetoric into state authority, institutionalizing invasion and sovereignty frames as legal declarations. This discourse-to-policy conversion creates a structural dilemma for the 2026 midterms: Republicans hold advantage while immigration remains framed as operational border control, but Democrats can reclaim initiative by shifting debate from whether sovereignty demands enforcement to whether enforcement respects democratic legitimacy, constitutional bounds, and international standing.