Vulnerable Democrats face an almost impossible choice: Side with Boeing and buck not only the White House but also ditch the unions that have long contributed to Democratic campaigns. But side with the unions and the federal government in a lawsuit challenging Boeing’s move into anti-union South Carolina, and you run afoul of Big Business in many Southern and Midwestern right-to-work states.

Moderate Democrats’ response for now: Run.

“I really don’t want to get involved,” said Sen. Mary Landrieu. The moderate Democrat from Louisiana has raked in more than $1 million in union contributions since her first election in 1996.

Sen. Ben Nelson, a Nebraska Democrat who faces a tough reelection bid in 2012, said he’ll start paying attention to the matter only if it comes up in a bill.

When Sen. Bill Nelson of Florida, a vulnerable Democrat up for reelection in 2012, stopped for a question from POLITICO on Tuesday, he bolted to the Senate floor when the case came up.

“I have to speak to a senator right now, I’m sorry,” Nelson said.

The National Labor Relations Board claims Boeing broke federal labor laws when it decided to open a $1 billion nonunion plant for its 787 Dreamliner airplanes in South Carolina because past union strikes at Washington facilities disrupted production.

If Democrats decide not to explain their stance, Republicans are happy to do it for them.

“It’s a tough position,” Sen. Lindsey Graham (R-S.C.), a vocal critic of the suit, told POLITICO. “With 9 percent unemployment, I can’t think of a better way to kill job growth than this complaint. And a lot of Democrats believe the unions have gone way too far. This is sort of toxic for them.”

Democrats “don’t want to upset the unions,” Graham added, “but at the end of the day, the American people need to hear from their elected representatives that we’re not going to drive people overseas.”

Nonetheless, a slew of moderates are staying quiet.

And Sen. Kay Hagan (D-N.C.) said she’s still studying the issue, though she’s a strong backer of the Tar Heel State’s right-to-work laws.

At an initial court hearing Tuesday in Washington state, where Boeing has its commercial airplane unit, an NLRB administrative law judge urged both sides to reach a settlement — to no avail. At an off-site congressional hearing Friday in Charleston, S.C., Republican Gov. Nikki Haley and NLRB’s top attorney are scheduled to testify before the House Oversight and Government Reform Committee, which is reviewing the complaint against Boeing.

Haley, elected last fall with strong tea party backing, is considered a powerful endorsement for presidential aspirants, especially given the fact that hers is an early primary state. And she has challenged candidates in her party to “step up” in the Boeing fight.

Some White House hopefuls did just that during a GOP primary debate Monday night in Derry, N.H., pledging to defend the nation’s 22 right-to-work states.

“We live in the United States of America, and people shouldn’t be forced to belong or be a member in any organization,” said former Minnesota Gov. Tim Pawlenty. “And the government has no business telling people what group you have to be a member of or not.”

“The right-to-work states are creating a lot more jobs today than the heavily unionized states,” added former House Speaker Newt Gingrich.

And businessman Herman Cain said the Obama administration — through the actions of the NLRB — is “killing our free-market system.”

Senate GOP leaders continued the assault against Obama and the NLRB at a news conference Tuesday, with Sen. Jim DeMint (R-S.C.) arguing that government interference will result in firms shipping jobs overseas. He’s rolled out legislation with Graham and Senate Republican Conference Chairman Lamar Alexander (R-Tenn.) that would bar the labor board from telling a firm it can’t move from one site to another.

“This unelected, unaccountable board is doing the bidding of union bosses and not protecting workers, either union or nonunion,” DeMint told reporters. “And we do call on the president to speak out. It’s time that America hears from you. Are you really for jobs in this country or jobs in another country?”

Republicans have vowed to block the confirmations of two of Obama’s nominees to the NLRB: Acting General Counsel Lafe Solomon and board member Craig Becker, a former union lawyer whom Obama granted a temporary recess appointment last year.

Graham on Tuesday repeated his threat to block Obama’s pick for commerce secretary, businessman and former Boeing board member John Bryson, until the president speaks out against the Boeing suit, which originated with the International Association of Machinists. Bryson stepped down from Boeing on Tuesday.

While the NLRB is an independent agency, the president does appoint its five board members. For the most part, the White House has been silent on the lawsuit, saying the board operates independently.

Senate Majority Leader Harry Reid has staunchly defended NLRB’s action, calling GOP attacks on the agency “disgraceful and dangerous.” And Democrats who represent heavily unionized states have voiced similar support.

“This is a legitimate case before NLRB,” Sen. Debbie Stabenow of Michigan, home of the nation’s automotive capital, told POLITICO.

But Iowa Democrat Tom Harkin, chairman of the Senate Health, Education, Labor and Pensions Committee, appeared exasperated when asked Tuesday about Boeing, and he declined to comment, even after speaking to reporters for several minutes about ethanol tax subsidies. While his is a right-to-work state, Harkin hasn’t taken sides in the fight. Instead, he seeks to correct Republican distortions.

“They have alleged that President Obama was behind this, that somehow because he has appointed a couple members of the National Labor Relations Board that he is behind this issue,” Harkin said during a floor speech in May. “President Obama has nothing to do with this whatsoever, and he should not have anything to do with it. But, again, people on the Republican side are alleging again, misinformation, misinformation, misinformation going out — that somehow this is being orchestrated out of the White House.”

Last month, Harkin joined nine other Democrats on his committee in sending a letter, urging NLRB to consider only the “facts and the law” — not politics — when it comes to pursuing the case.

But Hagan, a member of the committee, didn’t sign it. The first-term North Carolina Democrat said she is still researching the issue, trying to determine whether the NLRB has the authority to block Boeing from opening a plant in South Carolina.

“North Carolina is a right-to-work state, and we will remain a right-to-work state,” she told POLITICO. “So I’m concerned about the issue of not being able to come in, because we are open for business, and we want jobs in North Carolina.”

Alexander, the No. 3 Senate Republican, argued that the legal battle over the Boeing plant isn’t a parochial or partisan matter. It speaks to the broader issue of whether America will continue to manufacture things.

“Why would Volkswagen go to Michigan if it knew it could never expand in Tennessee or South Carolina or Georgia?” Alexander asked in a brief interview with POLITICO.

“Democrats, as well as Republicans, need to know that this is about whether Ford, General Motors, Volkswagen, Nissan, Boeing are going to make in the United States what they sell here. And this is just one more obstacle to that that makes it more likely jobs go overseas.”