"We invested in you, now it's your turn to invest in us," United Auto Workers' President Gary Jones said at a joint event with GM executives in downtown Detroit.
This year's talks on a new four-year contract between the union and the Detroit automakers are expected to be contentious, as US auto sales are slowing after a long boom.
Rising healthcare costs, job security and the use of temporary workers are also expected to be major sticking points.
GM in particular has been a target of union ire since announcing the closure of five North American plants late last year.
That move drew widespread condemnation, including from Republican US President Donald Trump. Criticism has persisted despite GM's stated efforts to seek jobs at other plants for all displaced workers.
Workers from GM's Lordstown, Ohio, plant, wearing red shirts emblazoned with "Invest in Lordstown," greeted UAW officials with applause as they arrived at the event on Tuesday.
GM is in talks to sell the Lordstown plant to electric vehicle startup Workhorse Group Inc and an affiliated, newly formed entity.
Workers in Lordstown built the Chevrolet Cruze, a sedan, sales of which had plummeted in the last few years as Americans abandoned passenger cars in favor of larger, more comfortable pickup trucks and SUVs.
GM Chief Executive Mary Barra said in remarks before a formal handshake with Jones that the pace of change in the auto industry has intensified as automakers invest heavily in the race to develop electric vehicles and self-driving cars.
"We are not here just to survive ... we are here to win," Barra said. Winning "represents growth and that means jobs," she added.
The union and Fiat Chrysler Automobiles NV (FCA) also formally launched talks on Tuesday at the automaker's Auburn Hills, Michigan, headquarters. In opening remarks, the UAW's Jones called on FCA to allocate new products to under-utilized assembly plants.
The UAW and Ford Motor Co officially launched negotiations on Monday, where Jones had struck a similarly adversarial tone, insisting that workers should share in automakers' profits and vowing to prevent their jobs being outsourced to Mexico or China.