With one week to go before the expiration of labor contracts between U.S. automakers and the UAW, union leaders are trying to negotiate the best deal possible for their members, knowing, however, that some of them will likely use the occasion to quit the union.

This is the first round of contract bargaining with General Motors GM +3.45%, Ford Motor and Fiat Chrysler Automobiles since Michigan's right-to-work law took effect in 2013. The law, which only applies to newly signed contracts, makes it illegal for employers to require workers to pay union dues as a condition of employment. Workers who choose not to pay dues lose the right to vote on union contracts, but the union is still legally required to represent them in conflicts with the company.

The stakes are high for the UAW, which depends on annual dues – about $550 per employee – to help fund its $214 million annual budget to represent members, pay staff and finance political and other union activities. The union has about $1 billion in assets.

UAW membership has grown in each of the past five years, to about 400,000 today, thanks to the resurgent auto industry and organizing efforts outside the auto industry such as higher education and gaming. Still, that's well below historic membership levels. Just a decade ago, the UAW had more than 650,000 members. Its peak was 1.5 million in 1979.

Michigan is the 24th state to pass a right-to-work law, but because the state is home to a majority of unionized autoworkers, it could put the UAW's growth trend in reverse.

That's why the pressure is on UAW bargainers to hammer out a good deal, especially while U.S. automakers are thriving. After the industry crisis in 2009, the UAW helped automakers significantly reduce their labor costs by accepting an unpopular two-tier wage system, which starts new hires at $15.78 per hour (rising to $19.28 in four years), vs. $28.69 for veteran production workers.