DETROIT | United Auto Workers President Ron Gettelfinger kicked off his union's bargaining convention Tuesday with a warning for companies: Just because we've cooperated in the past, don't take that as a sign of weakness.

In a speech at the start of the two-day national bargaining convention in downtown Detroit, Gettelfinger said the union will fight companies at the bargaining table, in politics and "if need be on the picket line."

Addressing 1,500 members from multiple industries who will help set the union's overall bargaining priorities, a defiant Gettelfinger said the union does not want to strike, but will if necessary.

"We will do what we have to do," he said. "Make no mistake about it. Collective bargaining is not collective begging."

The convention delegates, from more than 800 UAW locals in the U.S. and Canada, weren't expected to get into the nitty-gritty of what will be discussed with individual companies.

Still, crucial contract talks with the Detroit Three automakers loom. They begin officially this summer, although meetings already have started. The national UAW contract with automakers expires in September.

In his speech, Gettelfinger acknowledged that the UAW has faced continued challenges since its last bargaining convention in 2002, citing DaimlerChrysler AG's February announcement that it would consider selling its U.S.-based Chrysler unit.

"We have equity and hedge funds circling our head as never before," he said of potential buyers for Chrysler.

Gettelfinger accused the funds of "stripping and flipping" companies they buy.

"Our union is on guard to protect the best interests of our membership," he said.

But it was concessions of the past, and potentially of the future, that rankled some union members. Outside the convention center where Gettelfinger spoke, about 20 members and retirees carried signs in protest.

One delegate, Mike Parker, a worker at Chrysler's Sterling Heights assembly plant, made a motion to change Wednesday's agenda to set aside time for organizing local leaders to fight back against companies that are demanding concessions plant by plant.

"The problem is the locals are being left to bargain by themselves in dealing with the companies," he said in an interview.

The motion was shot down by a voice vote, but others made similar speeches about zero concessions and bringing an end to a two-tier wage scale in which new hires are paid far less than older workers.

One of the picketers, Martin Shawl, 53, a 28-year Delphi Corp. and General Motors Corp. worker from Bay City, said he doesn't believe the Detroit automakers are in financial trouble.

"It's voodoo accounting," he said, questioning the timing of the Chrysler Group's losses and GM's restatement of earnings due to accounting troubles.

He said the union shouldn't give back anything to the companies.

The UAW's main employers -- GM, Chrysler and Ford Motor Co. -- have lost billions during the past two years and are expected to demand major concessions from the UAW in contract talks this summer.

Among the issues are health care costs for active and retired workers, wages, work rules and the jobs bank, in which laid-off workers get most of their pay.

Gettelfinger said nothing directly Tuesday about potential concessions to auto companies, although he said the union would not respond to rumors or speculation from analysts and others about what the UAW should do.

He also called for universal health care for all, as well as for fair trade agreements.

"It would be a grave mistake to equate our actions to capitulation," he said of cooperation with employers in the past.

Although there were vocal calls to play hardball with the Detroit Three, others on the convention floor said concessions may be necessary in certain circumstances.

Darren Burgy, 32, a worker at a Jamestown Industries Inc. plant that supplies bumpers to a nearby GM plant in Lordstown, Ohio, said givebacks may be necessary to keep jobs.

GM has not said if the Lordstown plant will have a car to build when production of the Chevrolet Cobalt and Pontiac G5 ceases after the 2009 model year, and analysts say it is a big bargaining chip for the company.

"If there's no car in the next year, then there's no work for my plant, either," Burgy said. "We realize we're probably going to have to give a couple of things back."

Mark Caruso, who leads a UAW local at a former Ford and Visteon Corp. plant in Saline that is now part of a holding company awaiting sale or closure, said the union has to pick its battles.

"I would only be willing to give up what is necessary to have the company survive," he said.

Caruso and others are upset that companies pay bonuses to executives while losing money and then ask workers to give up pay or benefits.

Others called for the UAW to renew efforts to organize U.S. plants run by Toyota Motor Corp. and Honda Motor Co., where the UAW in the past has had little success. Gettelfinger recently announced a drive to do just that.

The convention comes as membership in the union and others nationwide continues to decline.

UAW membership peaked in 1979 at 1.5 million, but has been dropping ever since. The union said it had an average of 576,000 members in 2006, down from 598,000 in 2005.

Last year, the number of unionized workers in the U.S. fell by 326,000 to 15.4 million. Unions represented 12 percent of U.S. workers last year, down from 12.5 percent in 2005.

According to the U.S. Department of Labor, the union membership rate has dropped steadily from 20.1 percent of the work force in 1983.