AS LONG as elections have been held, candidates have sought to bend the rules to their advantage. American political parties have taken gerrymandering to new heights, using computer models that enable districts to be crafted block by block for maximum partisan gain. The Supreme Court is now taking notice, having accepted two cases that question whether it is constitutional for legislators to choose their voters, rather than the other way round. But Republicans, whose victories in 2010 put them in a position to doctor far more districts than Democrats have, are taking no chances. A change to the questionnaire for the decennial census in 2020 is expected to increase the share of districts whose voters prefer Republicans.