WASHINGTON - The National Labor Relations Board dropped its high-profile lawsuit against Boeing on Friday, but the political fallout continued as Republicans said the case was a mistake to begin with and a top agency official defended his conduct.

The board moved to end the case after the Machinists union approved a four-year contract extension with Boeing this week. Under the deal, the union agreed to withdraw allegations that the company built a nonunion plant in South Carolina to retaliate against past union strikes in Washington state. In return, Boeing promised to build the new version of its 737 airplane in Washington state.

"We've treated this case just like every case that comes to us, and it's ended the way most of our cases have ended," Lafe Solomon, the board's acting general counsel, told reporters in a conference call.

Solomon said he had always preferred a settlement and called the outcome typical at an agency that sees 90 percent of its cases settle.

Sen. Lindsey Graham, R-S.C., said it was hard for him to celebrate the dismissal of a case "which never should have happened." He called on Congress to investigate whether the union and the board collaborated against Boeing.

Solomon had come under intense criticism from Republican lawmakers, South Carolina officials and GOP presidential candidates for bringing the case. Republicans and business groups claimed the board was setting a dangerous precedent by interfering with legitimate business decisions about where to locate workers.

Solomon said Friday that 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. "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."

President Barack Obama, in response to a question from a reporter about the case, said simply: "I'm glad people are going to be working."

Obama had declined to take sides in the dispute because the labor board is an independent agency. In comments this year, the president said companies need to have the freedom to relocate, though they must follow the law when doing so. Obama had urged Boeing and the union to work out their differences.

But the president and the board had come under withering attack from GOP White House hopefuls running in South Carolina, an important early primary state, who pointed out the agency was run by Obama's Democratic appointees. Those attacks continued Friday, with former Massachusetts Gov. Mitt Romney taking the administration to task.

"Thanks to President Obama's appointees, the NLRB has become a rogue agency that tramples on the rights of American workers and businesses, injecting job-killing uncertainty into the economic climate," Romney said in a statement.

GOP presidential hopeful Newt Gingrich called the outcome a victory for right-to-work states. He condemned the lawsuit as "a politically motivated assault on the rule of law by President Obama and his big labor allies."

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, S.C., 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.

Graham said he would continue to place an indefinite Senate hold on nominations to the labor board as a way to keep it from making any more key decisions. Starting next year, the NLRB will have just two members instead of the usual five after a Democratic appointee's term expires. The board needs at least three members to rule on cases.