This article argues that the LDP manufactured its parliamentary dominance in postwar Japan by strategically altering specific facets of the electoral system. More generally, I demonstrate that intraparty politics play a crucial role in determining when and how electoral rules are changed. Despite widespread evidence that the LDP would win more seats under an SMP electoral formula, party leaders were repeatedly blocked from replacing the postwar MMD-SNTV system by intraparty incumbents, who feared that such a change would harm their individual reelection prospects. However, party leaders had greater leeway in altering rules that generated fewer intraparty conflicts. Between 1960 and 1990, the LDP implemented approximately fifty changes to campaign regulations, most of which were aimed at enhancing the incumbency advantage of all rank-and-file MPs. Statistical tests confirm that absent pro-incumbent revisions to the electoral code, the LDP would have succumbed to declining public popularity and lost its majority at least a decade earlier.