But at the same time, the court ruled that the Texas Legislature violated the Voting Rights Act in redrawing a particular district in southwestern Texas when it adopted the plan in 2003. The Legislature had carved up Laredo, removing 100,000 Mexican-Americans and adding an Anglo population from the Hill Country to shore up the faltering prospects of the Republican incumbent.
The decision means that a Federal District Court in Texas will now have to redraw the boundaries of that district and the surrounding ones. The district court is likely to act in time for the midterm Congressional elections in November, as it did almost exactly 10 years ago when faced with an earlier Supreme Court decision that called for a rapid response.
But it is not clear whether the change back to a Latino majority in the district would enable the Democrats to defeat the incumbent, Henry Bonilla, who received only 8 percent of the district's Latino vote in 2002, the court's decision said.
The ruling also cleared the way for other states to join Texas in adopting the approach that was challenged in the case: setting aside the tradition of redrawing Congressional districts only after the once-a-decade census, instead using a change of political control in the state governments as reason to reshape their maps. But there was no indication that there would be any rush to do so.
In political terms, the ruling was something of a vindication for Mr. DeLay, who stepped down from his leadership post and resigned from Congress this year after being indicted last year in Texas on charges of illegally routing campaign contributions to Texas Republicans. His indictment was related to his effort to win control of the Texas Legislature in 2002; it was that development that allowed the redistricting to go forward, helping Republicans to win six additional House seats in Texas in 2004.
With only Justice Anthony M. Kennedy joining both parts of the decision, the court looked in two directions in its most important voting rights case of the decade, rejecting the statewide gerrymandering claim brought by Democrats and other plaintiffs while accepting the Voting Rights Act challenge in southwestern Texas, brought by the Mexican American Legal Defense and Educational Fund. The case produced six separate opinions, a total of 123 pages.
On the gerrymander question, only two justices, John Paul Stevens and Stephen G. Breyer, found the Texas plan completely invalid, calling it a violation of "the state's constitutional duty to govern impartially."
Justice Kennedy's opinion for a plurality of justices — on two sections of the opinion he spoke only for himself — kept open the theoretical possibility that a partisan gerrymander might someday be found unconstitutional. But that prospect appeared remote. Despite finding that the Texas Legislature appeared to have acted "with the sole purpose of achieving a Republican congressional majority," Justice Kennedy said the case did not provide a "workable test" for deciding "how much partisan dominance is too much."
On the Voting Rights Act question, the majority's strong disapproval of what the Republicans did in southwestern Texas showed that the statute remains a crucial tool for minorities who can show that their right to equal participation in the political process has been impaired.
"In essence the state took away the Latinos' opportunity because Latinos were about to exercise it," Justice Kennedy said in his majority opinion.
The decision was based on Section 2 of the Voting Rights Act, one of the law's permanent provisions, which guarantees to minorities the right to "participate in the political process and to elect representatives of their choice." A separate provision, Section 5, is up for renewal and has become bogged down in Congress because of objections from some Republicans in the House.
The three-judge Federal District Court in Austin that had upheld the plan in its entirety must now redraw the district lines in southwestern Texas before the November election.
Although the Texas attorney general's office said in an official statement that only "one district must be partially redrawn," any changes would have ripple effects in neighboring districts, and the court's opinion clearly contemplated a broader remedy. "The districts in South and West Texas will have to be redrawn," Justice Kennedy said, without specifying a number or procedure.
Justice Kennedy was joined in the Voting Rights Act part of the decision by the court's four most liberal members, Justices David H. Souter and Ruth Bader Ginsburg along with Justices Stevens and Breyer. This was, in fact, the only part of the decision that five justices signed.
Justice Kennedy's rejection of the statewide gerrymander challenge, brought by Texas Democrats and others, had the support of a majority of the court for his conclusion, but not for his analysis.
Justices Antonin Scalia and Clarence Thomas agreed because they believe, as they said in a case from Pennsylvania in 2004, that claims of partisan gerrymandering were categorically invalid and could never be considered by a federal court.
Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr. took no position on whether such claims could ever be brought. With that proviso, they said Justice Kennedy had resolved the issue correctly in this case, League of United Latin American Citizens v. Perry, No. 05-204.
Justices Souter and Ginsburg agreed with one relatively minor part of Justice Kennedy's analysis, his rejection of the argument that a mid-decade plan that relied on census data from the beginning of the decade, not taking account of inevitable population shifts, violated the constitutional requirement of one person one vote.
But on the more general question of how to assess a redistricting plan for impermissible partisanship, these two justices said there was "nothing to be gained" by revisiting an issue on which the court was deadlocked. They said they would keep the issue alive for future cases but would not express a view now.
With only Justices Stevens and Breyer voting to invalidate the Texas plan as an invalid gerrymander, that left the vote on the gerrymander part of the opinion at 5-to-2-to-2.
The Texas plan was adopted in 2003 after a protracted struggle during which Democratic legislators fled the state to deprive the Legislature of a quorum. It replaced a plan drawn by the federal court in 2002 when the Republican-controlled Senate and the Democratic-controlled House failed to agree.
The court-ordered plan was greatly resented by the Republicans, who viewed it as unfairly carrying forward the 1990 plan, drawn by the Democratic-controlled Legislature, despite the fact that Republicans had made statewide gains in the intervening decade. So when the Republicans picked up the State House in 2002, they made redistricting a priority. Success was immediate. The Congressional delegation went from 17 Democrats and 15 Republicans in 2002 to 11 Democrats and 21 Republicans after the 2004 election.
The plan dismantled the districts of several Democratic incumbents. In one portion of the opinion on Wednesday, the court rejected a challenge brought on behalf of black voters to the redrawing of a Dallas district long represented by a white Democrat, Martin Frost, who had black support. Justice Kennedy said that with only 25.7 percent of the population, African-Americans were not numerous enough to assert that their "effective control" of the district had been taken away.
Of the many strands to the complex case, the court's treatment of the southwestern Texas district was perhaps the most surprising. Chief Justice Roberts wrote a strongly worded dissenting opinion, which Justice Alito signed. "It is a sordid business, this divvying us up by race," he said.
His dispute with Justice Kennedy was over how to evaluate the challenged district in light of the fact that the Legislature had at the same time created a new district with a Latino majority, running a narrow band, 300 miles long, from Austin to McAllen, on the Mexican border. Chief Justice Roberts said this district meant that over all, the Latino vote was not impermissibly diluted under the plan.
But the Kennedy majority said that this new district was illegal under the Voting Rights Act and could not offset the loss of the Laredo district. He said the district was not sufficiently compact and did nothing more than combine "two far-flung segments of a racial group with disparate interests," whose "only common index was race."