The janitors who walked off the job in the Twin Cities Wednesday night and Thursday morning were forced by economics and the evolution of American labor to resort to what appears to be a weak weapon: a strike that lasted just one day.

The janitors and their allies had the power of disruption and surprise on their side. They were able to shut down two interstate ramps in downtown Minneapolis Thursday morning, turning the morning commute chaotic for some downtown workers and creating a three-mile backup on northbound Interstate 35-W.

But it was a sharp contrast to the strident, sometimes violent, prolonged walkouts that for decades characterized the labor movement.

The one-day strike is being seen again and again across the country. Teachers in Detroit held a sickout on a day when President Obama was in the city last month to call attention to poor working conditions. Baggage handlers, cleaners and janitors at seven major U.S. airports staged a one-day walkout at seven airports the week before Thanksgiving.

They result from the reality that inflicting long-term economic pain on employers is impossible for most low-wage workers, who justifiably fear the threat of being replaced and don't have the means to go without a paycheck for weeks or months. They hope instead that a short strike can generate publicity, seize public support, galvanize workers and force employers' hand at the bargaining table.

"We do one-day strikes because our members are low-wage workers," said Javier Morillo, the president of Local 26 of the Service Employees International Union, which represents about 4,000 janitors in the metropolitan area.

The cleaning companies who employ the janitors have all week highlighted the temporary nature of the strike, and the fact that bargaining will continue on Feb. 22.

"Now that the union has ended its 24 hour strike, the employers will be able to concentrate on our ongoing contract negotiations instead of strike preparations," said John Nesse, a lawyer and chief negotiator for the cleaning companies.

Morillo said the strike worked despite its brevity, and that Local 26 is not ruling out a longer strike.

"More people are thinking about the janitors in their building today than a week ago," he said. "If it has to get bigger, it will get bigger."

But he also makes no secret of the fact that the strike was in large part a publicity event.

"Every strike, every labor action, is both about PR and the actual impact," he said.

The burst of activity on Wednesday and Thursday, which included other advocacy groups allied with Local 26, was an attempt to bring others into the discussion besides the two parties at the bargaining table, said John Budd, who teaches labor relations at the University of Minnesota's Carlson School of Management. The goal with a short strike is to grab the maximum amount of attention from anyone who can exert pressure on the cleaning companies who are negotiating with the union.

"The janitors are working in buildings serving people who they're technically not working for and technically not negotiating with," Budd said. "If they're cleaning at Target headquarters, Target isn't the employer. Target's probably trying to stay out of it. SEIU probably hopes that this actual disruption, or threat of disruption, creates some pushback from the tenants who actually occupy these buildings."

Thanks to a 1980s shift in corporate social norms, attributed by labor historians to President Reagan's toughness in handling the Professional Air Traffic Controllers Organization strike of 1981, more employers are willing to replace striking workers than they were 40 years ago. That history looms over labor strategy and helps limit unions' options.

"The first day that you are on strike, you can literally be replaced by the employer," said Laura Cooper, a labor law professor at the University of Minnesota. "If I'm an engineer, and I go on strike, it's a pretty good likelihood that they're not going to find an engineer to replace me tomorrow, but if I'm a janitor and I go on strike today, I can be replaced and I may never get that job back."

The law gives a replaced striker preference if a job opens later, but over time the worker's circumstances and the job often change in ways that make reinstatement unlikely.

A one-day strike is a way to demonstrate solidarity to employers and signal that an economic strike could happen.

Unions at factories in the 1970s and 1980s were made up of members who went to work together in the same place every day. They were colleagues and friends.

For a union like Local 26, whose janitors often work alone or in tandem in office buildings sprinkled across the Twin Cities, a strike is a unifying event. On Wednesday, janitors who work in different buildings and different cities marched and sang together in front of U.S. Bank Plaza.

"It serves to rally the membership and build solidarity," said Budd.

Given that attention from the public and news organizations is critical to the success of these strikes, Budd said one-day strikes must be used sparingly.

A good analogy comes from the opening to the musical "Newsies," he said. The lead headline in the morning paper that day is "Trolley Strike Drags On for Third Week." Reading that, the newsboys are not pleased. They sing together at the men chalking up headlines, "Hey, stupid, that ain't news no more!"

And it's not just reporters and television news producers who might lose interest in a third or fourth janitor's strike. The workers themselves might lose faith if they don't see results, Budd said.

"Nothing came of the first one, nothing came of the second one, why should I bother?" Budd said. "If this doesn't really change the dynamics at the bargaining table, how many more times can you do it, I think, is an open question."

Adam Belz • 612-673-4405 Twitter: @adambelz