The Constitution says (Article I, Section 2) that the number of Representatives (members of the House) shall be determined every 10 years, based upon population. The states then decide on the districts, and how and when this is done isn't in the Constitution. There is an obligation to follow the one-person, one-vote concept.

The Texas Democrats gerrymandered like mad in 1990. The Republicans, who were the majority party in Texas by 2000 (based on state-wide elections), tried to do the same then (when the state got 2 more representatives) but the 1990 gerrymander denied them a majority in both houses of the state legislature. The Federal Court imposed a redistricting when the legislature couldn't agree. The 2002 state elections gave them a majority in both houses and they passed their redistricting plan. What the Supreme Court basically said is that (1) there is nothing in the constitution specifically forbidding mid-decade redistricting and (2) this, being the will of the elected representatives, properly supercedes the court-ordered plan imposed in 2000. Gerrymandering itself is enshrined in American political tradition.

The part of the redistricting that was thrown out specifically involved a district that had been majority Latino that was changed to reduce that majority. This was determined to be a violation of the Voting Right Act.