As voting by Ford (F) - Get Report union workers enters its last day, a tentative four-year labor contract is in danger of rejection. Workers are showing their bargaining strength at a moment of prosperity in the U.S. automotive market. 

More than half of the 34,000 Ford workers who have voted on the contract negotiated by the United Auto Workers union turned it down. Another 8,000 at Ford plants in the Dearborn, Mich. area will vote Friday, with the outcome uncertain. 

Interviews with workers at Ford plants in Kansas City, Mo., and Louisville, Ky., which manufacture the automaker's pickup trucks, suggested that dissatisfaction stems from the rate at which newer workers will progress to the top wage scale. Some workers hired since 2007 could wait eight years before earning nearly $29 an hour. 

"If I'm turning a wrench on South Chassis, and I've got an entry level on east Chassis I want them making the same wage," said Todd Dunn, president of UAW Local 862 in Louisville. 

Other union officials and labor market experts caution that Ford workers could be trading a richer contract, assuming they are able to negotiate one with the company, for future union jobs that could be foregone as assembly work migrates to non-union plants in Mexico and even China

"Workers think this is a pretty healthy time, this is a pretty healthy company," said Kristin Dziczek, a labor analyst with the Center for Automotive Research in Ann Arbor, Mich. "If they're not going to recoup all the concessions they've made in the past now, when are they going to recoup them?" 

The U.S. vehicle market is on track to notch between 17 million and 18 million sales this year, the best since 2000. 

Ford shares are off about 6% year to date, compared with a Dow Jones Industrial Average that is roughly flat. 

Dunn said he wasn't surprised by the rejection, since Louisville workers realize their leverage in negotiations is potent, given the fact that they build Ford's highly profitable pickup trucks. With those profits on the line, the company may be less likely to risk a walkout by holding firm on wages. 

Shortly before the financial collapse of the industry in 2008-2009, the UAW agreed to create a second, lower-paid wage tier for new workers in order to relieve pressure on General Motors (GM) - Get Report  and Chrysler. GM and Chrysler eventually filed for bankruptcy. The UAW agreed not to strike until the expiration of the latest contract in September. 

Dziczek noted that Detroit-based automakers are looking to Mexico for assembly of passenger cars, which command a lower profit margin and thus are vulnerable to above-market wage rates. 

Jimmy Settles, the UAW's chief Ford negotiator, told the Milwaukee Journal he "understands members' frustration, but he has to keep Ford competitive." Younger workers, he said, don't understand the union's policy of seeking wage parity among the Detroit-based automakers. 

UAW workers at Fiat Chrysler (FCAU) - Get Report rejected one proposed contract before accepting a second, sweetened pact. GM hourly workers accepted a contract, but rejection by skilled trades means an agreement hasn't yet been reached.

Doron Levin is the host of "In the Driver Seat," broadcast on SiriusXM Insight 121, Saturday at noon, encore Sunday at 9 a.m.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.