WASHINGTON -- The Supreme Court gave politicians legal license Wednesday to aggressively redraw election districts to benefit the party in power, as it upheld the mid-decade redistricting plan engineered by former House Majority Leader Tom DeLay and other Texas Republicans.

By clever line-drawing, DeLay and the Texas Legislature -- with both houses newly under GOP control in 2003 -- remade its delegation in Congress, turning a 17-15 Democratic majority into a 21-11 Republican majority in 2004. The bold move signaled an escalation in partisan warfare.

Before, the redrawing of electoral districts had been a once-a-decade battle that followed the release of new census numbers. Under the Constitution, states are required to adjust district lines to account for population changes.

Wednesday's ruling means they may redraw the lines whenever they choose, as long as they do not violate voting rights laws. Legal experts and political strategists said the ruling would encourage Republicans in other GOP-dominated states to redraw their districts to gain more seats.

It is not clear whether Democrats will be able to do the same. In the ruling, the court emphasized that the Voting Rights Act generally forbade splitting up blocs of minority voters. That makes it harder to create more Democratic districts.

Mary G. Wilson, president of the League of Women Voters, called the decision "extremely disappointing," saying it would encourage politicians to become serial mapmakers. "We now can expect an even more vicious battle between the political parties as they redraw district lines every two years for partisan gain," she said.

The partisan nature of redistricting has inspired efforts to take the process out of the hands of lawmakers. Last year, California Gov. Arnold Schwarzenegger proposed to give a panel of retired judges the task of redrawing electoral districts, but voters rejected the idea.

Gerrymandering is hardly a new phenomenon. The word came from an 1812 cartoon that portrayed a district drawn by Massachusetts Gov. Elbridge Gerry as resembling a salamander.

In recent decades, computers have given politicians an ever more powerful tool to shape the outcome of elections by shifting voters among districts. In the past, the Supreme Court has struck down "racial gerrymandering" and said the Constitution generally bars officials from making decisions based on race.

Politics is another matter, and although many Supreme Court justices have voiced unease over brazenly partisan gerrymandering, they have never struck down a redistricting plan as too partisan.

On Wednesday, a five-member majority said DeLay's plan, even if it were drawn for a purely partisan purpose, did not violate the Constitution and its guarantee of equal protection under the law. But the court did find one district to be illegally drawn because it diluted Latino voting power.

The overall ruling applies to other electoral districts as well, including those for state legislatures. Electoral districts are usually drawn by politicians in state capitals, the justices noted, and it is hard to say when such a politically drawn plan becomes too partisan.

Justice Anthony M. Kennedy, speaking for the court, cited two other reasons for upholding the Texas plan.

First, he said, the task of redistricting belongs to elected legislators. [emphasis added to the original]. In 2001, a panel of federal judges redrew the Texas districts because the state Legislature -- then divided between a Republican Senate and a Democratic House -- could not agree on a plan. So the plan pressed by DeLay in 2003 was the first to win the approval of the state's elected legislators.

"There is nothing inherently suspect about a legislature's decision to replace mid-decade a court-ordered plan with one of its own," Kennedy said. Second, he said, it is not clear that DeLay's plan was less fair than the Democratic-friendly plan it replaced. [emphasis added to the original]

Before 2003, Democratic leaders had used their power in Austin to preserve a Democratic majority in the state's congressional delegation, even as most Texans voted for Republicans. Four years ago, 59% of Texans voted Republican and 41% Democratic in statewide tallies, yet more Democrats than Republicans won election to the House of Representatives.

By this measure, DeLay's plan "can be seen as fairer" than the one it replaced, Kennedy said.

The court's four conservatives -- Chief Justice John G. Roberts Jr. and Justices Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr. -- joined Kennedy in rejecting the charge of "partisan gerrymandering" against the Texas Republicans.

At the same time, Kennedy joined with the four liberal justices -- John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer -- to rule that one southwest Texas district was drawn illegally in a way that hurt Latino voters.

The Voting Rights Act of 1982 forbids officials from "diluting" the power of minority voting blocs, and Kennedy said Texas lawmakers violated that provision when they shifted 100,000 Latino voters to shore up the reelection prospects of Rep. Henry Bonilla, a Republican who has been unpopular with Latino voters. The Texas Legislature split Webb County, which is 94% Latino, between Bonilla's 23rd District and a neighboring district.

This ruling requires the Texas Legislature -- or perhaps a panel of judges -- to redraw that district, which will force others to be changed. The court's decision to uphold the mid-decade redistricting plan, despite its objections to how one district was drawn, was the second defeat for liberal reformers this week.

On Monday, the court rejected a move by a group of reformers to lessen the impact of money in politics. They had strongly defended a novel Vermont law that set low spending caps for state candidates, but the Supreme Court, in a 6-3 ruling, struck it down as a violation of the 1st Amendment.

Good-government activists also joined the Texas case, hoping it could lead to competitive election contests that would be decided by the voters, not by the politicians who drew the districts. Typically, politicians draw districts that have a comfortable majority of Republicans or Democrats, so the outcomes are essentially foregone conclusions.

Ken Mehlman, chairman of the Republican National Committee, called the ruling "a victory for Texas voters and the Republican Party." He added that it "confirmed that the current Texas map is fair under any standard."

J. Gerald Hebert, a lawyer for the Texas Democrats pressing the case, said he was dismayed the court had turned a blind eye to an "extreme example of raw partisan politics." The decision "opens the floodgate for partisan redistricting," he said. "The court has essentially ceded the field and state legislatures have largely been given a free hand to do what they will with congressional districts."