Striking United Auto Workers members and supporters attend a speech by Vermont Sen. Bernie Sanders outside General Motors’ Detroit-Hamtramck Assembly plant on Sept. 25, 2019 in Detroit.Michael Wayland / CNBCDETROIT – As the United Auto Workers and General Motors continue negotiations to potentially end an 18-day strike against the automaker, the head of the union’s Ford Motor unit says “significant progress” has been made on those talks.In a letter to Ford’s unionized workforce Thursday, UAW Vice President Rory Gamble said 18 out of 20 of the subcommittee bargaining units have reached tentative agreements ahead of “large economic items” being discussed with top-level negotiators once the talks move to Ford.The union’s negotiations with GM have been slow moving, taking a full week to resolve smaller issues before they could move to the top negotiators, or “main table.”While the UAW selected GM as its “target” company to establish a “pattern” agreement that will be used at Ford and Fiat Chrysler, the union’s non-GM departments have continued negotiations with their respective companies.”While the UAW-GM bargaining team continue to set the pattern by negotiating first, we remain ready and able to complete our negotiations when called upon,” Gamble wrote.Ford’s time could come sooner rather than later if negotiations with GM fall apart or the union decides it could get a better deal with the remaining outstanding issues at Ford. While not routinely done, the union has switched or threatened to switch the target company in the past. However, that’s not normally done while workers are on strike.A person familiar with the talks said the union’s focus remains on the current patterned bargaining with GM. A UAW spokesman declined to comment on the potential of the union moving on to Ford before reaching a deal with GM.Ford, in response to the letter, reiterated a previous statement that its “focus is reaching a fair agreement with the UAW that allows the company to be more competitive so we can continue to preserve and protect good-paying manufacturing jobs and maintain our track record of investing in our U.S. plants.”Negotiations between GM and the UAW are still hung up over the company’s use of temporary workers and the path for hourly workers to achieve top wages, among other interwoven issues, according to people familiar with the negotiations.Gamble also said the union’s Ford department is developing a “transparent and factual” plan for educating members about the deal once one has been reached.Any potential deal needs to be voted on by the UAW’s local union leaders and then rank-and-file members. That process typically takes two weeks, but it could be expedited if the union decides to keep workers on strike during the voting.Ratification of tentative agreements traditionally hasn’t been a problem. However, workers with Fiat Chrysler four years ago rejected the initial deal approved by UAW leaders — which sent negotiators back to the table.Ahead of the current negotiations, industry analysts cautioned that the ratification of any agreement could be challenging as a federal corruption probe reaches into the UAW’s highest ranks.