JEFFERSON CITY, Mo. - Not content to target only public-sector unions, Republican lawmakers in more than one-quarter of the states this year launched their most ambitious attempt in about generation to break up union shops in factories, offices and other private-sector workplaces.

But their efforts have largely faltered, in part because of opposition from fellow Republicans concerned that pushing too hard against unions could jeopardize other aspects of the pro-business agenda they staked out after strong statehouse gains in the 2010 elections.

Of the 14 states where "right-to-work" bills barring mandatory union fees were considered, only New Hampshire has passed the legislation, and it is uncertain whether Republican lawmakers can overcome an expected veto by the Democratic governor.

In Indiana, which had been hailed as the best hope for right-to-work advocates, Republican Gov. Mitch Daniels implored GOP lawmakers to back off over fears that the anti-union measure would derail other parts of his agenda. Republicans also have balked at right-to-work bills in Maine and Missouri, where the top Republican senator has pledged to make one final push against long odds before the session ends May 13. In many other states, similar bills never even made it out of committees.

Some Republican lawmakers who represent more heavily unionized areas have teamed up with Democrats to protect unions from the legislation. Others simply don't believe the assertion from GOP colleagues that limits on private-sector unions will propel businesses to locate or expand in their states and thus drive down unemployment rates.

"There's a lot of things the state can do to improve the business climate," said freshman Republican Sen. Mike Kehoe, the owner of a central Missouri car dealership and service center where employees have twice rejected unionization efforts. But when it comes to right-to-work legislation, "I don't think it's the magic wand that does anything right now."

Twenty-two states, predominantly in the South, already have right-to-work laws that prohibit union fees from being a condition of employment. Elsewhere, unions can reach contracts with businesses that require fees to be deducted from everyone's paychecks to help cover union bargaining costs.

Most state right-to-work laws were passed in the first couple of decades after a new Republican-led Congress in 1947 enacted greater federal restrictions on union powers. Since the 1970s, just three states have enacted right-to-work laws - Louisiana in 1976, Idaho in 1985 and Oklahoma in 2001.

During that time, private-sector union membership has plunged by more than two-thirds nationwide, to slightly less than 7 percent of the work force last year.

The decline in union membership, recent Republican gains in many state legislatures and continued high unemployment combined this year to provide right to work advocates with both an economic justification and the potential political momentum to roll back union powers.

"It's an exciting time because it seems like the energy for right-to-work is growing here in Missouri and across the country," Mark Mix, president of the National Right to Work Committee, said during an early March visit to Missouri Capitol.

A few days later, with scores of union members watching from the galleries, the Missouri Senate took up the bill - only to set it aside after three hours of debate during which some Republicans made it clear they would join Democrats in a filibuster. Republican Senate President Pro Tem Rob Mayer has vowed "to make a strong run at it" again before the session ends.

The right-to-work push in Missouri - a historically solid union state - may be the strongest since 1978, when voters defeated a ballot measure.

The chief argument in today's movement is that Missouri, New Hampshire - and other states that allow mandatory union fees - are missing out on jobs from major manufacturers that prefer not to deal with unions.

When New Hampshire gave final approval to a right-to-work bill on Wednesday, Republican House Speaker William O'Brien declared it the "single greatest opportunity to create jobs in New Hampshire that the Legislature will pass this year."

Those arguments gained some credence a few weeks ago when the National Labor Relations Board accused Boeing Co. of illegally retaliating for a 2008 union strike in Washington by adding a non-union assembly line for its new 787 passenger jet in the right-to-work state of South Carolina. That came after Boeing announced last year that it was moving 500 jobs from California to the right-to-work state of Oklahoma.

Consultants that help businesses locate potential new sites said state laws on union powers can sometimes make the difference.

"If you've got two locations that on paper map out equally, you're looking at opening a new factory and one is right to work and one is a non-right to work, overwhelmingly the client is going to choose the non-union state," said Chris Schastok, a site consultant for the Chicago-based commercial real estate firm of Jones Lang LaSalle.

But other factors often rise to the forefront, including the skills and education of the local labor force, the proximity to needed supplies and state and local tax incentives.

"Most companies historically in the U.S. have used the right-to-work position as a barometer of business climate. I would not say that is the fact in the last 10 years," said Mike Mullis, president and CEO of JM Mullis Inc., a Memphis, Tenn.-based site location firm.

Mullis said he testified before legislative committees in support of Oklahoma's right-to-work law about a decade ago. The results of that law have been mixed - and open to interpretation.

The Oklahoma Chamber of Commerce touts the fact that the state skyrocketed from 40th to first in the nation in new job creation in the first six months after passage of the right-to-work law. Yet the liberal-leaning Economic Policy Institute notes that the change in Oklahoma's unemployment rate over the past decade is virtually no different than that of its six surrounding states, half of which do not have right-to-work laws.

Republican state Rep. Tom Winsor, of Maine, had sponsored right-to-work legislation several years ago that was defeated in a Democratically controlled Legislature. With Republicans now holding a slim majority in the Maine House, Winsor ramped up for another attempt this year. So far, he's been disappointed.

"I don't think any Democrats support the bill. Republicans, I think they're divided. So at this point, I don't think it will proceed," Winsor said.

Associated Press writer Glenn Adams contributed to this report from Augusta, Maine.