They clean and guard buildings that house some of the Twin Cities' largest corporations, which means the unprecedented decision Saturday by a union of 6,000 janitors and security officers to allow negotiators to call a strike as soon as this week could have widespread repercussions.

"It could basically shut down the whole metropolitan area," said security guard Paul Keith, who works at Minneapolis' Retek Building. "They can't operate without us working."

On Saturday, about 500 members of the Service Employees International Union (SEIU) approved allowing negotiators to call a strike if they can't reach agreements with employers -- contract maintenance firms for buildings such as Target Corp., Wells Fargo Center, Medtronic and U.S. Bancorp Bank Plaza. It would be the first time both groups have had a strike; the two contracts have never expired at the same time and the only strike was a daylong one by security officers five years ago, according to the union.

"We're in a serious moment for everyone here," said U.S. Rep. Keith Ellison, D.-Minn., one of several politicians at Saturday's event. "I'm so proud of you for your courage."

It was disappointing, though, to John Nesse, chief negotiator for nine janitorial contractors, including ABM, Marsden and Harvard Maintenance. "We continue to remain hopeful we'll reach an agreement with the union," he said, adding that four meetings are scheduled, starting Monday.

In a letter from the Minneapolis-St. Paul Contract Cleaners Association to its customers, the group said it's committed to finding an "agreement that is fair and equitable, and one that is also mindful of the current economic challenges," but added that the union hasn't changed positions on increases in wages or health benefits.

In security officer contract negotiations, a federal mediator has been brought in. David Duddleston, a spokesman for the security contractors-- including American Security, AlliedBarton and Securitas -- said that the contractors are bargaining in good faith and that they have two meetings this week with union negotiators.

Alfredo Estrada, who has worked at Bloomington's Minnesota Center without a contract since Jan. 1, said a strike is the last resort, but is needed to show they want fair wages.

"It's a small sacrifice for a bigger victory in the future," Estrada said in Spanish. "What they're offering is a joke. They're proposing to set us back 15 years."

He and more than 1,000 janitors' full-time jobs could be reduced to part time, cutting benefits and health insurance that he, his wife, and his 7- and 5-year-old daughters depend on. "They've left us with no other choice," he said. "We deserve more."

Kelly Smith • 612-673-4141; Twitter: @kellystrib