Rep. Randy Forbes (R-Va.) could be in danger of losing his seat if a new congressional map for Virginia stands. (Lawrence Jackson/AP)

Republican lawmakers in Virginia want Supreme Court Chief Justice John G. Roberts Jr. to stop a new congressional map from taking effect until after the 2016 elections.

The Supreme Court is planning to review a federal panel’s ruling that the state’s congressional map, drawn by a Republican legislature, illegally packed black voters into the district of Rep. Robert C. Scott (D) to dilute their overall influence. The map approved by that panel would move some of Scott’s reliably Democratic voters into Rep. J. Randy Forbes’s district, making it harder for the Republican to win reelection. It also shifts voters in three other districts.

The ruling immediately moves more than 1 million people in parts of central and southeastern Virginia. On Tuesday, the Republicans involved in the lawsuit asked the high court not let that happen before the justices rule.

Should the Supreme Court find that the previous map was constitutional, they wrote, the result would be “electoral chaos, mass voter confusion, an immense waste of taxpayer and private resources,” and possibly a postponement of the election itself. If the ruling comes after the June 14 primaries, they added, the damage would be even worse — new primaries would have to be scheduled.

“No matter how one handicaps the likelihood of reversal, it is far too high to expose the Commonwealth,” the Republicans wrote.

The possibility of a reversal also forces candidates to campaign and collect signatures in both potential districts, they noted.

The three-judge panel of the U.S. District Court of the Eastern District of Virginia previously ordered the General Assembly to redraw the congressional map, but after a special session ended with no action, Gov. Terry McAuliffe (D) asked the judges to take over.

One judge on the three-person panel broke from his colleagues in the 2014 ruling, saying that a remedial map was “neither required, nor permitted.”

The map the panel chose most profoundly affects Forbes, putting the heavily black cities of Petersburg and Richmond in his territory. His district follows a diagonal path between Chesapeake and western suburbs of Richmond.

But Rep. Dave Brat (R), who unseated Eric Cantor, then the House majority leader, in a 2014 primary, would also lose some of his most conservative supporters in a district just north of Scott’s and Forbes’s. Brat faces a primary challenge from a more moderate Republican, Henrico County Sheriff Mike Wade. Reps. Scott Rigell and Robert J. Wittman, both Republicans, get slightly more friendly seats as all of Newport News moves into Scott’s district.

Of Virginia’s 11 representatives, eight are Republicans and three are Democrats.

Under the map ruled unconstitutional, which was drawn in 2012, Scott’s district went from a voting-age population that was 53.1 percent African American to one that was 56.3 percent African American. Under the new map, that proportion drops to 45.3 percent while the black voting-age population in Forbes’s district rises from 31.3 percent to 40.9 percent.

Forbes said in a statement that he was “confident that the Supreme Court will conclude that the overreach . . . is not appropriate.”

The challenge to Virginia’s congressional district map and similar ones around the country were brought by attorney Marc E. Elias and funded by the National Democratic Redistricting Trust. Elias is general counsel to Hillary Clinton’s presidential campaign and worked for McAuliffe’s campaign. Elias could not be reached Wednesday for comment.