Mitt Romney’s harsh critique Monday of the federal government’s efforts to block Boeing from bringing thousands of non-union jobs to South Carolina was not a new line of attack for Republican presidential candidates who for months have used the conflict to portray the Obama administration as beholden to union bosses.

But the moment did provide a chance for the former Massachusetts governor to take a position fully in step with his party’s conservative wing. Primarily because of his support for a health-care overhaul in his state that became the base for President Obama’s national plan, Romney has struggled to win over tea party activists, whose energy and uncompromising stance on reducing the size and scope of government have redefined the Republican Party.

“It’s the most egregious example of political payback, where the president is able to pay back the unions for their hundreds of millions in political contributions at the expense of American jobs,” Romney said of the National Labor Relations Board’s decision to sue Boeing for trying to move thousands of jobs from unionized facilities in Washington state to a new $1 billion facility in South Carolina, a right-to-work state.

“It’s an assault on business,” Romney said, even going so far as to use the term “union stooges” to describe Obama’s appointees to the NLRB. “It’s an assault on jobs. It’s an assault on states that have right-to-work policies. It’s simply the product of political payback. And frankly, it should not be part of our political system. It’s unseemly.”

If Romney pleased Republicans in South Carolina and beyond with his sharp political rhetoric, his attack also underscored how important such moments may be for a presidential hopeful who has not backed down from his moderate views on other issues. He is walking a fine line between selling himself as the most electable candidate in the GOP field and avoiding alienating the conservatives whose blessing he needs to secure the party’s nomination, particularly in such early states as Iowa and South Carolina.

Social Security is perhaps the best example, and Romney has promised to try to fix, not abolish, the entitlement program. He has portrayed his chief rival, Texas Gov. Rick Perry, as out of step with most Americans for calling it a “Ponzi scheme” at risk of imminent collapse.

Even in his attack on unions, Romney took pains to reach out to workers while blaming the leaders of their labor groups for standing in the way of new jobs and economic growth. Surrounded by huge projected slides and signs laying out his plan to improve the economy, Romney took aim at union leaders for using workers’ dues to funnel political contributions to Democrats, for trying to end the use of secret ballots on unionization votes and for demanding political payback — with the Boeing deal, for example — from the leaders they elect. Standing beside him was his latest supporter, former Minnesota governor Tim Pawlenty, who recently bowed out of the GOP race.

“We believe in the union movement,” Romney said after touring Boeing’s enormous facility in North Charleston, where the company plans to build its new 787 Dreamliner passenger airplane. “We have no problem with unions. We have the most productive workforce in the world. The output per person in America leads the world.”

But, he added, “unions shouldn’t be used to pursue the political agendas of their bosses.”

Romney wouldn’t be the first Republican to reach out to pro-labor voters in a presidential campaign even as he takes the party line against the organizations themselves. Former president George W. Bush did so successfully in 2000. And with many union leaders saying that Obama has done too little for labor, Romney has good reason not to slam the door in labor’s face.

The Boeing/NLRB controversy has become a touchstone for Republican candidates this year. At issue is the labor board’s allegation that Boeing was retaliating against its unionized workers in Washington by trying to move the 787 plant — and thousands of jobs — to South Carolina. Retaliation over union activities is illegal under federal labor laws. But critics of the NLRB argue that the board’s efforts represent a flagrant encroachment on free enterprise and that Boeing has a right to have factories wherever it chooses.

Jim Davis, a retiree from the Charleston area who identifies with the tea party’s principles and came to listen to Romney on Monday, said he hasn’t decided whom to support. But he called Romney’s stance on Boeing a “no-brainer.”

“What it will come down to is: Can we have a candidate who adheres to a significant number of conservative principles and can also win?” Davis said. “Perfection doesn’t exist.”

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