The National Labor Relations Board has formally dropped its legal challenge against Boeing over a nonunion plant the aviation giant opened in South Carolina -- a move met with jeers by Republicans who long described the case as "frivolous" and anti-business. 

The decision, announced Friday by the board's top attorney, comes after the Machinists union approved a 4-year contract extension with Boeing. As part of that deal, the union agreed to withdraw its charge that the company violated federal labor laws. 

"This is the outcome we have always preferred," Lafe Solomon, the board's acting general counsel, said in a statement. 

The NLRB counsel had originally challenged Boeing's decision to open the South Carolina plant, with the union claiming the company did so in retaliation for past strikes in Washington state. Under the recent deal, Boeing promised to build the new version of its 737 airplane in Washington state. 

But South Carolina lawmakers, as well as GOP presidential candidates, piled on the NLRB over the case, describing it as a politically motivated attack at the behest of union bosses that should never have happened. 

"For the sake of the Boeing South Carolina workers, I'm pleased to hear the frivolous complaint that has put a cloud over their operations has been lifted," Sen. Lindsey Graham, R-S.C., said. 

Sen. Jim DeMint, R-S.C., said those with the NLRB who led the case should still "resign immediately." 

DeMint said the "damage" had already been done, claiming "a precedent has been set by the NLRB that they will attack businesses in forced-unionism states that try to create jobs in right-to-work states." 

On the presidential campaign trail, several GOP candidates offered similar derision. 

Former House Speaker Newt Gingrich described the suit as a "politically motivated assault on the rule of law by President Obama and his big labor allies." 

Former Massachusetts Gov. Mitt Romney described the NLRB as a "rogue agency." 

But White House Press Secretary Jay Carney stressed that Obama was not involved in the NLRB proceedings. "The NLRB is an independent board," Carney said, adding that Obama is "glad" the two parties reached a resolution. 

The agency settles about 90 percent of its cases. 

Solomon said Friday he was simply following the law and might do it again if faced with similar facts. 

"This case was never about the union or the NLRB telling Boeing where it could put its plants," Solomon said in a conference call with reporters. "This was a question for us of retaliation, and that remains the law. If we were ever faced with a similar pattern, we might well issue a complaint." 

Solomon said the Boeing complaint was always about the loss of future jobs in the Seattle area. 

The new deal between Boeing and the union resolves that issue, because there is now job security in Seattle and job security at the North Charleston plant, he said. Both sides say the agreement ushers in a new era of labor peace after years of acrimony. 

Boeing spokesman Tim Neale said the company continues to believe the complaint was without merit and should have been dropped. 

"Boeing is grateful for the overwhelming support we received from across the country to vigorously contest this complaint and support the legitimate rights of businesses to make business decisions," Neale said. 

The Associated Press contributed to this report.