How do commodity booms influence political fragmentation in weak-party, clientelist regimes? Existing literature suggests resource windfalls typically empower incumbents by increasing voter support or facilitating patronage expansion. However, I argue that in regimes where parties are weakly institutionalized and clientelist linkages predominate, booms instead undermine incumbents and fuel fragmentation. Using Brazil’s First Republic (1889-1930) as a case, I analyze eleven congressional elections across eighteen autonomous state party systems. Employing a difference-in-differences approach, I find positive commodity price shocks decrease incumbent vote shares, heighten fragmentation, and enhance success rates for new parties. These effects are amplified in states with homogeneous export economies, as diverse economic interests strengthen existing party attachments. District-level analysis confirms increased party proliferation in boom-affected areas, while candidate-level data shows incumbent politicians are more likely to seek reelection with new parties. This study highlights conditions under which economic booms, rather than busts, destabilize elite coalitions and reshape political institutions.