On Aug. 31, as tensions between the city and union heightened, 50 Detroit bus drivers were suddenly afflicted with illnesses that kept them away from work that day. “That’s at least 50 bus drivers we don’t need,” responded Detroit Mayor David Bing.

His tone, cooler than refrigerator temperature, conveyed the message that Detroit’s newest mayor might act more like Ronald Reagan — who once fired the nation’s striking air traffic controllers — than like any of his mayoral predecessors.

Similarly, Detroit school Emergency Financial Manager Robert Bobb is seeking concessions from the city’s unionized teachers as he confronts a $300 million deficit that he says could lead to bankruptcy. Bobb especially wants to reform seniority rights so the district can put more experienced teachers in academically challenged classrooms and help rejuvenate the ranks with younger teachers.

This is a sore point with the Detroit Federation of Teachers, which has one month left in a two-month extension to negotiate a new contract with Bobb.

For decades, Detroit’s dozens of labor unions have wielded fearsome clout in the city, despite the illegality of public union strikes and the Motor City’s steadily declining population, fortune and work force.

That legacy of union might — from their heyday in the 1960s through the administration of Mayor Coleman Young, a labor organizer turned politician — is likely to deteriorate further when Detroit labor unions this fall face a maelstrom of economic calamity, from slashed budgets to the burden of retiree health care and pension costs, from local pressures to national economic trends.

Finding the bottom — of wages, job losses, benefits — is the challenge that negotiators on both sides of the table face. But it is the union leaders whose historical place is under siege. They are being asked to give back the gains of a half-century, trying to salvage jobs and a standard of living that’s no longer sustainable.

“These are difficult times for unions,” says Hal Stack, director of the Labor Studies Center at Wayne State University. “The only question is whether that will be through job elimination or salary and benefit cuts.” Already, wages have been cut 10 percent for nonunion workers.

With a $300 million deficit for the city of Detroit, declining revenues and the prospect of municipal bankruptcy looming like a guillotine blade, not even labor union chiefs dispute the city’s dire financial status or the delicate status of their defenses.

Labor leaders from Al Garrett, president of Michigan AFSCME, which represents 3,500 Detroit workers, to David Hecker, who heads the Michigan Federation of Teachers, are clear about the array of forces lined up against them, even as they are reluctant to capitulate.

Their power has been diminished in the last year by a cascade of factors that give the city of Detroit’s leaders and Bobb, the state-appointed manager, unprecedented openings for change in the work force and rules that govern employees. Among these are:

• The UAW’s contracts since 2007 shattered precedent by cutting wages and benefits. While lauded by many as courageous, they also signaled a new era of declining resources and the inevitability of concessions.

“For much of the 20th century, the UAW set the standard,” says Nelson Lichtenstein, a labor history professor at the University of California/Santa Barbara and author of an acclaimed biography of Walter Reuther. “Today it’s a totally shrunken force.”

• Detroit unions used to command true political power: The yeah-or-nay on candidates for office. But since the mid-1990s, city workers have been legally free to live outside the city limits. Increasingly, teachers and city employees are suburbanites who can’t elect — or recall — city officials.

Now Bing, facing re-election, is willing to alienate even the city’s most powerful union, AFSCME. Garrett withdrew support from Bing on Sept. 10, and nine other unions switched earlier this week.

• The threat of bankruptcy is particularly chilling to union leaders, because it creates an opportunity for even long-standing union contracts to be voided. Both Bing and Bobb at Detroit Public Schools have discussed bankruptcy as a real, last-gasp option.

• Bing and Bobb were explicitly hired — in one case by voters, in the latter by Gov. Jennifer Granholm — to change the status quo. Both have expressed more interest in that role than becoming popular or long-term politicians.

Bing faces a Nov. 3 election — but he’s repeatedly styled himself as a one-term mayor interested in saving the city, not placating interest groups.

“Our reality is that we cannot continue to spend what we do not have,” he wrote in an August commentary for The News.

Then there’s Bobb, who was hired on the strength of his reputation for financial acumen and decisiveness.

It is, in short, “a terrible time right now, the very lowest spot” for labor unions, says Erica Kimble, whose Detroit service workers union, SEIU, represents 2,500 janitors and maintenance workers. (“My phone rings if the public sector takes deep cuts,” she says).

“In three of our last few contracts, the employers have specifically pointed to the UAW, asking for similar concessions.”

Says Stack: “Now the public-sector folks are saying, ‘Look at the UAW. Those new workers aren’t going to have retiree pensions, they have lower starting pay, they have defined retirement plans.'”

And union leaders recognize that no provisions are sacrosanct: Everything from paid vacation days to health care is back on the table.

Bankruptcy looms as an especially troublesome option because it would allow the city and/or school district to discard existing labor contracts.

“I think it would be a major negative for everybody,” says Hecker, the president of AFT Michigan, the state affiliate that includes the Detroit Federation of Teachers. “I don’t think there’s a need for it. We would much rather reach agreements.”

In the crush downward, it’s easy to forget that unions helped forge Detroit’s middle class, pioneered health care benefits and provided for workers’ retirements. Real or inflation-adjusted wages doubled — between 1947 and 1973 — as unions ascended, and helped equalize pay for women and African-Americans, especially in Michigan.

How much of that historic record is now likely to crumble?

Stack of the Labor Studies Center describes a political and ideological assault on public sector unions. After job cuts, benefits become the next resource, “because there’s a limit to how much you can downsize.”

Reaching a happy medium is easier when both sides can gain.

“What labor asks for is fairness,” says Hecker, who reports “good ongoing discussions” concerning the Detroit teachers. He is less enthusiastic about treatment of support workers, who have gone without contracts and raises for years. “All school district staff — teachers and support staff — should be treated fairly and with respect.”

Already, though, the city’s non-unionized employees have accepted 10 percent pay cuts. In the new reality born of deficits, “we’re going to become more creative,” as Hecker puts it.

When Dennis Archer looks back at his two terms as mayor, he remembers going to New York and negotiating an improved bond-rating with Moody’s.

“I had something to work with then,” he says. “Dave Bing doesn’t have anything to work with.”

Laura Berman is a Detroit News columnist. Contact her at lberman@detnews.com