Four years ago, the United Automobile Workers agreed to allow Chrysler, G.M. and Ford to pay lower wages to new hires to help close the cost gap with foreign carmakers. Now the two-tier arrangement is at the forefront of labor talks between the U.A.W. and the Detroit companies.

The union’s president, Bob King, has made an increase in entry-level wages a top priority in negotiations for a new national contract to replace the current agreement, which expires Wednesday.

So far, about 12 percent of Chrysler’s 23,000 union workers earn the lower wage, and over all, 4,000 or so of the 112,000 U.A.W. members are second-tier hires. Those numbers are expected to grow — and in fact can increase significantly even under the current contract. The jobs are central to the contract talks because they are viewed as critical to the industry’s continued recovery.

Some benefits for the lower-tier workers are scaled back as well. They get the union’s traditional medical benefits, but a maximum of four weeks paid time off a year, versus five for the longtime workers. And instead of the guaranteed $3,100-a-month pension a full-paid worker receives after age 60, the new hires have to build their own “personal retirement plan” based on contributions from the company of less than $2,000 a year.

The gap in wages between regular and entry-level workers has created dissent in U.A.W. ranks. Some long-term employees have demonstrated against the two-tier system and called for it to be abolished. Mr. King, however, has focused on getting meaningful pay raises for the lower tier rather than eliminating it.

At the big Labor Day parade in Detroit, union activists chanted “equal pay for equal work,” and some full-paid workers said they were willing to forgo a wage increase in the new contract to help the lower-tier employees.

“In order to get those guys up, we’ll take a signing bonus or profit-sharing instead,” said Gary Wurtz, a line worker at G.M.’s plant in Orion Township, Mich., where 40 percent of the employees are lower tier.