Maryland Gov. Martin O'Malley, left, shows Speaker of the House of Delegates Michael Busch a pen that he used to sign a congressional redistricting bill. (Patrick Semansky/AP)

Maryland’s General Assembly on Thursday approved — and Gov. Martin O’Malley signed into law — a partisan plan designed to pick up another House seat for Democrats by anchoring most of the state’s eight congressional districts in suburban Washington, where minority populations are surging.

Most dramatically, the plan stretches a rural Western Maryland district held by the state’s senior Republican lawmaker nearly 200 miles from the border of West Virginia to the Capital Beltway in Montgomery County to pick up African American, Asian, Hispanic and other reliably Democratic voters.

Across the state, the new lines divide minorities among multiple districts, preventing the creation of a new, third congressional district dominated by minorities. Maryland Republican leaders and a grass-roots group that unsuccessfully urged that such a district was natural in part of Montgomery County called on the U.S. Justice Department to investigate whether O’Malley (D) and Democrats racially gerrymandered Maryland’s congressional map for their party’s gain.

As a result, the day’s developments in Annapolis may have marked the end of the political battle over the state’s redistricting but only the start of legal ones.

“We believe the governor has violated the Voting Rights Act, and we ask the Justice Department to look at what the state of Maryland has done and to decide,” said Radamase Cabrera, a community activist with the Fannie Lou Hamer Political Action Committee.

For nearly 50 years, the Voting Rights Act has mostly been used to force states to weigh if districts are drawn to allow blacks to pick the candidates of their choosing. But with an increasingly diverse population, it is morphing into an ever more complicated and nuanced test of minorities’ voting rights, election law experts say.

Fast-growing Hispanic and Asian communities as well as more mingling of minority and white neighborhoods are forcing states, federal courts and the Justice Department to consider whether minority voters in different corners of the country have more in common with one another than with white voters and, therefore, if they should be lumped together for elections.

Redistricting litigation is pending in 22 states, and in nearly half of those — including many of the largest states critical to control of the House of Representatives — arguments about how to treat amalgamations of African Americans, Hispanics, Asian Americans and other minorities are expected to become the focal point of courtroom battles, experts say.

“This is only going to become a bigger and bigger issue as the country grows ever more diverse,” said Justin Levitt, an elections law professor at Loyola Law School in Los Angeles and who runs the Web site All About Redistricting, which tracks legal challenges in the 50 states. “The legal claim is going to be that either the groups were drawn together didn’t deserve it, or the groups that were split up didn’t deserve that, either.”

It’s an issue that Virginia and most other Southern states automatically come under scrutiny for because they are required to submit all redistricting plans to the Justice Department for review.

Maryland is not required to, and it is in another class of states that until this year have been involved in relatively few cases on the topic. But a standard legal test exists: Could minorities make up a voting majority? Do they have a history of voting together? And do non-minorities tend to vote against candidates favored by minorities?

In Maryland, there are many different opinions on all three questions, but central will be whether it was unconstitutional for O’Malley and Maryland Democrats to split up newly majority-minority Montgomery. The map slices the county into three of the state’s eight congressional districts, each connected to more rural areas. Whites make up at least 63 percent of each district.

Rep. Donna F. Edwards (D) and Montgomery County Council President Valerie Ervin (D) were among those who in recent weeks claimed that the map tramples on minorities’ voting rights.

Edwards said that the map would allow minorities in Montgomery to help elect Democrats but prevent them from successfully electing another African American or any other minority to Congress. Her arguments were undercut, however, because she was seen as acting in self-interest to regain the share of Montgomery she lost under the new map.

In a letter dated Thursday and released after the General Assembly approved the plan, Attorney General Douglas F. Gansler (D) wrote that the map met the office’s usual standard for state laws of “not clearly unconstitutional.”

He elaborated that his office found no reason to believe that the congressional redistricting plan “constitutes a racial gerrymander” in violation of the 14th Amendment. He also released an analysis with the same conclusion by the state’s independent redistricting expert, Bruce E. Cain, a political science professor at the University of California at Berkeley.

Cain said that he analyzed Maryland voting patterns from the 2008 Democratic presidential primary and general election and that the results showed that Hispanic and African Americans voted differently in the primary, with blacks more likely to support Barack Obama in both elections.

But Levitt argued that big national races are not the most telling when it comes to how minorities would choose to vote. Legal tests, he said, are focusing increasingly on local races, such as school boards and judicial races, in which a minority candidate may stand a better chance and be able to demonstrate that they can build a coalition of support from across racial and ethnic communities.

Del. Ana Sol Gutierrez (Montgomery), who was one of five Washington area Democrats who voted against the plan in the House of Delegates, said any legal test should look at races such as those of County Council member Nancy Navarro, who lost an election in 2008 but won a special election in 2009 after gaining support from heavily African American districts and others.

Del. Kumar P. Barve (D-Montgomery), the first Indian American elected to a state legislative body, disagreed.

“We’re past race as in issue Montgomery County,” he said.

Sen. Brian E. Frosh (D-Montgomery) didn’t go that far but said “It’s not like Montgomery County is free of bias, but maybe we’re as close as it gets in the United States.”

Senate Minority Whip E.J. Pipkin (R-Queen Anne’s) said Democrats were missing the point on whether Montgomery was a utopia. “You’re disenfranchising minorities. Look at the census data: It says the bulk of the state’s population increase is minorities, and this map divides that growth up. That’s a good court case.”

In the arms race that redistricting has become, Pipkin said Democrats went too far.

Legislative leaders left little doubt that their top goal is to unseat 10-term Rep. Roscoe G. Bartlett (R). His traditionally conservative 6th Congressional District in Western Maryland will plunge into Montgomery and go from having none of the state’s top minority precincts to having 11 of the top 25 Asian ones, as well as tens of thousands more blacks and Hispanics.

Statewide, the map will give Democrats a leg up to win all but one of the state’s seats next year in Congress, even though four in 10 Marylanders regularly vote Republican in national elections.

The man who may ultimately decide Maryland’s map is Thomas E. Perez, a former member of O’Malley’s Cabinet and now assistant attorney general for Justice’s Civil Rights Division.

Perez also had to gain support from a variety of Montgomery minorities and white voters to win the election that started his political career. In 2002, he beat a crowded primary field to become the first Hispanic member of Montgomery’s County Council.

In a speech on redistricting earlier year to the National Association of Latino Elected and Appointed Officials, Perez was clear about his mandate. “Substantially unequal districts or vote dilution cannot be justified,” he said.

Database editor Ted Mellnik contributed to this report.