"The biggest reason [for the program's suspension] is that auto companies begin and end promotions based on sales demand," Adams explained. "And we're in a period with the auto manufacturers, certainly in the case of GM, where they are producing vehicles in response to very high demand and they are relying less on incentives like the credit union member discount. They have discontinued some other incentive programs as well."

Indeed, GM reported Tuesday that its retail deliveries increased by 6% compared to a year ago, when results included the Labor Day holiday. GM's Chevrolet, Buick, GMC and Cadillac brands sold more than 270,000 vehicles in the U.S. in August 2015.

"GM's retail sales increase far outpaced the industry in August, and we have grown our retail share for five months in a row compared to last year," Kurt McNeil, GM's U.S. vice president of sales operations in Detroit, said in a statement.

What's more, the automaker continued to have the highest average transaction price and posted the lowest incentives as a percentage of average transaction prices in the U.S. auto industry, according to GM's statement.

GM's average transaction prices in August totaled $34,000, up by more than $300 month over month and about $660 year over year. In August, GM's incentive spending was 12% of average transaction prices, down by 0.8% from July 2015, while the industry average incentive spending was up by 0.2%.

"All of the economic fundamentals that we look at, including job growth, disposable income and fuel prices, are in good shape and that should keep sales strong," McNeil said.

Although thousands of credit unions from around the country participated in the program, about 900 credit unions, including many of the nation's largest cooperatives, were involved in the program's premium level.

However, Adams noted that GM made the program available to all credit union members nationwide regardless of their credit union's level of promotional activity.

Adams described the GM discount program as hugely successful for credit unions because it promoted the value of credit union membership and helped cooperatives stand out in highly competitive auto loan markets.

CU Solutions Group pitched the idea of the credit union member discount program to GM and Chrysler in late 2008.

The GM discount program was launched in January 2009 when the North American auto industry was on the verge of imploding, in part, because of the effects of the global financial crisis and subsequent Great Recession.

"This program was about cars and loans, but in 2009, it was also about helping to save the domestic auto industry, millions of jobs and the financial well-being of so many families," Adams said. "Credit union leaders should pause and be thankful for this unprecedented service opportunity that contributed so much in such a profound way."

Adams noted that he described the GM program as being suspended rather than discontinued because he hopes the Michigan league may be able to "re-engage with GM and get the program going again sometime in the future."

"We will continue to look for opportunities with other automakers," he said. "We are open to talking to other automakers, and we are in the process of seeing whether domestic and foreign auto companies might have an interest in a similar program."