Answer: Gerrymandering typically benefits whichever political party has the majority in the legislature at the time the congressional district lines are redrawn.
Context / history:
The landmark case regarding voting district lines was Baker v. Carr (1962), which pertained to voting districts in Tennessee. The plaintiff, Charles Baker, argued that voting districts, which had not been redrawn since 1901, heavily favored rural locations over urban centers which had grown significantly since then. Joe Carr was Secretary of State for Tennessee at the time, so was named in the case in regard to voting district lines as drawn by the state legislature. The Supreme Court ruled that voting districts were not merely a political matter to be decided by legislatures, but that they were subject to review by federal courts to determine their fairness.
The matter of redrawing district lines has come up in court cases again recently as some state legislatures, when dominated by one political party, have "gerrymandered" district lines to try to maintain continued prominence for their party. In the present political context, this has given the Republican Party an advantage in the overall voting balance in the country.
