U.S. Rep. Robert C. “Bobby” Scott, shown in 2014, is Virginia’s lone African American congressman. (Earl Gibson III/Getty Images)

The Supreme Court on Monday takes up a long-running political fight about whether Virginia lawmakers redrew the state’s congressional map to protect the commonwealth’s lone African American congressman — or to make sure he was not joined by a second.

The court will consider whether Republican lawmakers packed African American voters into Democratic Rep. Robert C. “Bobby” Scott’s district to comply with the Voting Rights Act or to make surrounding districts more hospitable to white candidates.

A lower court ruled against the legislature last year, and the judges then created a second district designed for a black candidate. Voters in the state’s congressional primary go to the polls in June — about when the justices would seem likely to rule on this new plan.

The case presents what has become familiar litigation over how states divide up their residents into congressional districts, which is essential to the country’s politics and crucial to political parties.

But state lawmakers charged with the task compare it to walking a tightrope, or crossing a minefield, or preparing a meal for Goldilocks.

Under the Supreme Court’s somewhat hazy guidance, state lawmakers must consider the race of those who populate each district to ensure minorities have a fair shot at being represented. But lawmakers fail if they let race become the predominate issue in drawing the lines.

“You can’t let the porridge be too hot, you can’t let the porridge be too cold,” said former Virginia delegate Bill Janis (R-Glen Allen), who devised the commonwealth’s congressional map. “But they won’t tell us what temperature the porridge has to be.”

The Supreme Court will take another shot at that in the Virginia case. A panel of federal judges said the commonwealth’s plan veered from partisan gerrymandering aimed at protecting incumbents — for which the Supreme Court has shown a high tolerance — into racial gerrymandering, which the Constitution forbids.

In a sense, said Nathaniel Persily, an election-law expert at Stanford Law School, “the Voting Rights Act is on a collision course with the Constitution.”

The questions for Virginia and other states are, he said, “how much can you think about race in construction of districts, and is the use of race in aid of partisan gerrymandering problematic?”

The line is increasingly elusive, especially across the South, where blacks are presumed to be Democrats and whites are increasingly Republican.

Because of the high stakes, redistricting plans in more than three-quarters of the states have been challenged in court.

But wherever the line is located, Virginia’s Republican-led General Assembly crossed it, according to an amicus brief to the Supreme Court filed by the Campaign Legal Center and League of Women Voters, among others. The legislature’s plan “sanctions the impermissible use of race as a proxy to achieve partisan gains,” the groups wrote.

Janis, along with current and past Republican congressmen who are challenging the lower court’s decision, contends the plan adopted after the 2010 Census simply preserved the status quo by retaining the cores of the state’s existing congressional districts. To meet obligations under the Voting Rights Act to protect minority-majority districts, it increased the percentage of the black voting-age population in the 3rd District in southeastern Virginia, long represented by Scott.

“Core preservation and incumbency protection were . . . the most important neutral principles and consistently applied to all districts,” Washington lawyer Michael A. Carvin, representing the Republicans, told the Supreme Court in a brief.

The redrawing of Scott’s district “was neither race-based nor a departure from neutral principles,” Carvin wrote.

But Democrats said that just a look at the “bizarrely shaped district” is enough to show other motives.

The district started north of Richmond, slid down the north shore of the James River, jumped over James City and formed a horseshoe around Newport News. That was only the beginning, as it included some towns and skipped others, until the black voting-age population of the district reached 56 percent.

Two residents of the district, Gloria Personhuballah and James Farkas, filed suit. They said the Republican-controlled General Assembly was not protecting Scott, who won his last race with more than 80 percent of the vote. Instead, they charged, legislators were making it easier to elect whites in surrounding areas by packing African Americans into Scott’s district.

Drawing legislative and congressional districts is key to a party’s continued dominance in a state. Republicans controlled Virginia when it redrew congressional and legislative maps after the 2010 Census. The party still controls the General Assembly, and eight of its 11 members of Congress are Republicans. This is in a state that hasn’t elected a Republican to statewide office since 2009, where both U.S. senators are Democrats and where voters twice chose President Obama.

The judicial panel, on a 2-to-1 vote, found fault with the General Assembly’s plan because it required that at least 55 percent of the voting-age population in Scott’s district be black. The prevailing judges said there was no evidence that such a high percentage was needed to comply with the Voting Rights Act and that setting this figure was contrary to a Supreme Court decision last term that sided with challengers to Alabama’s legislative redistricting.

The Supreme Court’s 5-to-4 ruling in the Alabama case said legislatures should not set arbitrary minority population goals for districts but instead look at what makeup is necessary to preserve the minority’s ability to elect a candidate of its choice.

This decision is one of several reasons to think Virginia Republicans have an uphill battle at the Supreme Court.

For one, there is a question about whether they should even be there. The Obama administration, backing Personhuballah and Farkas, says that because Virginia did not appeal the lower court’s decision, it should stand. The Republican lawmakers do not have standing to challenge the ruling simply because it changes their districts, Solicitor General Donald B. Verrilli Jr. told the court in a brief.

The congressmen claim they are harmed because a new plan “will move ‘unfavorable Democratic voters’ into the districts they represent, thereby decreasing their chances of reelection,” Verrilli’s brief states. The court should “reaffirm that voters in our democratic system choose their representatives — not the other way around.”

Another bad sign for the challengers is that the justices turned down Carvin’s request to put a hold on the lower court’s plan to impose a new congressional map. The court’s action followed the General Assembly’s failure to provide its own remedy. Carvin argued that it would be wrong to allow the primary election to go forward when there is a possibility the new districts would be thrown out.

The new map shows changes in line-drawing can have a dramatic impact on a state’s politics.

The judges’ plan drastically reduced the number of black voters in Scott’s district, which is still considered safe for him. It increased the number of black voters in the 4th District, which is represented by Rep. J. Randy Forbes (R).

So, Forbes announced he will instead run for the 2nd District seat being vacated by Rep. Scott Rigell (R).

The newly drawn 4th District is a better fit for state Sen. A. Donald McEachin (D-Henrico), an African American who is chairman of the Senate Democratic Caucus and ran for attorney general in 2001. McEachin filed paperwork to run for the seat and wasted no time in kicking off his campaign three days after the legislative session adjourned.

If the Supreme Court upholds the lower court’s decision — and even a tie among the eight justices would do that — law professor Persily said Virginia might provide a real-life answer to theoretical musings about how changing district lines affects results.

“The election is going to prove who’s right,” he said.