Negotiators for the UAW and General Motors met into the evening Saturday as the autoworkers' national strike wrapped up its sixth day.

A separate strike by UAW-represented janitors who work for Aramark and serve five GM sites in Michigan and Ohio was in its seventh day.

Two top Democratic presidential candidates announced plans to visit the picket line at Detroit-Hamtramck Assembly Plant.

Sen. Elizabeth Warren of Massachusetts said she would be at the factory at noon Sunday, which coincides with the union's "Solidarity Sunday." The union invited the public to join GM and Aramark strikers, and union chaplains will speak at noon.

Sen. Bernie Sanders of Vermont said he would visit the same plant on Wednesday. Sen. Amy Klobuchar of Minnesota, a candidate toward the bottom of the Democratic presidential field, visited Detroit-area strikers earlier in the week.

Former Vice President Joe Biden planned to visit a labor rally Sunday near Kansas City, Kansas, the union said.

The strikes

The strikes started last weekend. First, about 850 janitors employed by Aramark and represented by the UAW walked off the job early Sunday. Then, after the union's separate 2015 contract with GM expired, about 46,000 GM workers went on strike across the United States early Monday morning.

At issue for GM strikers are wages, health care benefits, job security, ending a tiered wage system and making temporary workers into permanent workers.

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Aramark workers have been working without a contract since March 2018. The janitorial jobs start at $11 and after five years rise to $15.18 an hour. They described some of the work they do as dangerous, using extremely high-pressure water to clean equipment in the paint shop, for example.

People close to GM and the UAW say it is important to reach a tentative new contract with Aramark around the same time as reaching one with GM. Otherwise, UAW workers for either side face the possibility of crossing the others' picket lines.

Read more:

UAW dealing with 2 strikes at once — and resolving them separately could get complicated

The GM offer

GM released a statement last Sunday saying it made a strong offer to the UAW that, "improves wages, benefits and grows U.S. jobs in substantive ways and it is disappointing that the UAW leadership has chosen to strike at midnight tonight. We have negotiated in good faith and with a sense of urgency. Our goal remains to build a strong future for our employees and our business.”

GM has said in its offer it also offered solutions for "unallocated" assembly plants in Michigan and Ohio. That's in reference to GM's announcement last fall that it would idle two transmission plants, one in Warren and one in Baltimore; along with Lordstown Assembly in Ohio and Detroit-Hamtramck Assembly.

GM's proposed solutions are thought to include building an electric pickup and making batteries at Detroit-Hamtramck and a battery cell manufacturing facility in part of Lordstown. GM continues discussions to sell Lordstown to an investment group that includes electric truck maker Workhorse.

The company's offer said it would invest $7 billion over the four years of the contract and offer 5,400 jobs. The Free Press has learned only half of those jobs would be new.

GM also offered an $8,000 ratification bonus. That's what it paid in 2015 to each member after the rank-and-file ratified that contract. GM paid a lump sum ratification payment of $2,000 to active temporary employees who worked at least 90 days prior to the effective date of the agreement.

But the Free Press learned that GM has proposed that workers pay 15% of their health care costs, up from the current estimated level of 3% of health expenses. Another person familiar with talks said GM's offer preserves current health care benefits at the same cost.

GM said it will spend $900 millio in 2108 on health care for U.S. hourly workers, which some industry observers consider unsustainable. The average U.S. worker pays about 28% of health care costs, according to the Center for Automotive Research in Ann Arbor.

Also, the Free Press reported that the offer included a 2% wage increase for UAW workers for the first and third year of the four-year contract and 2% lump sum payments the second and fourth years. The 2015 contract increased wages 3% in the first and third year with a 4% lump sum increase the alternate years.

Workers, whose pay has regressed 16% against inflation since 2010, say it's their turn to be rewarded after years of profits for GM and the other automakers. They also want to equalize pay for workers hired after 2007, who start at $17 per hour and can rise to about $28 per hour after seven years.

Another key issue involved temporary workers. About 7% to 10% of GM's workforce over the course of a year is made up of temps, who are paid $15 an hour and lack the opportunity to transfer if their plant closes. The union wants a path for them to become permanent; GM wants the right to hire more temps for flexibility.

The union chose to negotiate first with GM in an effort to reach a template agreement that it can then take to Ford and Fiat Chrysler Automobiles, which are operating under contract extensions.

Ripple-effect layoffs

The effects of the UAW strike against GM are being felt by thousands of suppliers that have temporarily laid off workers because GM assembly plants are not accepting deliveries.

On Monday, if the strike continues, GM will idle part of its DMAX engine plant in Moraine, Ohio, said GM spokesman Dan Flores. It builds the 6.6-liter turbo diesel engine in GM’s heavy-duty pickups, a product of Flint Assembly Plant. About 550 workers at DMAX, which are represented by the International Brotherhood of Electrical Workers, will be temporarily laid off. DMAX will continue to build engine blocks, but not full engines.

Unifor, Canada's autoworker union, said Friday that 4,500 of its members have been temporarily laid off, CNBC reported. That includes 1,200 workers at GM's truck assembly plant in Oshawa, Ontario, which was shut down late Tuesday because of a shortage of parts made at UAW-represented U.S. plants. On Friday morning, Oshawa’s car line stopped due to a parts shortage caused by the U.S. strike, GM said. About 2,000 hourly employees are now on temporary layoff at the car plant, which is scheduled to close at the end of the year.

Oshawa’s stamping operation continues to run. It stamps sheet metal for Ontario's CAMI plant, which builds the Chevrolet Equinox SUV. Unifor told CNBC that GM plans to temporarily lay off 700 additional workers at the company’s St. Catharines plant Monday and will “re-evaluate” production at its CAMI plant next week if the strike continues. A GM spokesman declined to comment on speculation.

GM has said its plants in Mexico remain operational but it is monitoring the situation daily.

Contact Eric D. Lawrence: elawrence@freepress.com or 313-223-4272. Follow him on Twitter: @_ericdlawrence.Read more on General Motors and sign up for our autos newsletter.