Service Employees International Union President Mary Kay Henry called the ad an unfair attack on an agency that protects working people. Romney "sharpened his weapons and went on a familiar attack against the 99 percent" of Americans who are not among the wealthiest one percent, Henry said in a statement.
In the ad, which hit the airwaves in South Carolina this week, Romney said the NLRB was "un-American" in challenging Boeing's choice to place a jet production line at its new non-union plant in South Carolina instead of union facilities in Washington State.
The agency filed the complaint in April on behalf of union workers in Washington, who had alleged Boeing was retaliating against them for previous strikes. Boeing denied the charges.
The complaint was withdrawn last month after the union and Boeing reached a contract extension agreement that settled the matter.
"The National Labor Relations Board, now stacked with union stooges selected by the president, says to a free enterprise like Boeing, 'You can't build a factory in South Carolina because South Carolina is a right-to-work state.'
That is simply un-American. It is political payback of the worst kind," Romney said in the ad.
The candidate was echoing the complaints of business groups that said the complaint would have a chilling effect on employers' expansion plans.
This week Obama used controversial recess appointments to bypass Senate approvals and install three new members to the NLRB's board -- two of them Democrats.
Boeing, which did not participate in the creation of the ad, declined to comment about it. A spokeswoman in Washington State for the International Association of Machinists and Aerospace Workers union, which spurred the complaint against Boeing, called the Romney ad "false."
"The fact is the NLRB helped the Machinists Union and Boeing resolve our conflict and created a platform for long-term relationship building between the company and the union," said spokeswoman Connie Kelliher.