WASHINGTON (Reuters) - U.S. President Donald Trump, who is engaged in a trade war with Beijing, said on Friday that the largest U.S. automaker, General Motors Co GM.N, should begin moving its operations back to the United States.

FILE PHOTO: The GM logo is seen at the General Motors Warren Transmission Operations Plant in Warren, Michigan October 26, 2015. REUTERS/Rebecca Cook

“General Motors, which was once the Giant of Detroit, is now one of the smallest auto manufacturers there. They moved major plants to China, BEFORE I CAME INTO OFFICE. This was done despite the saving help given them by the USA. Now they should start moving back to America again?” Trump said in a post on Twitter.

Trump appeared to be referring to a Bloomberg News story that reported GM's hourly workforce of 46,000 U.S. workers has fallen behind that of Fiat Chrysler FCHA.MI as the smallest of the Detroit Three automakers. Over the past four decades, GM has dramatically cut the size of its overall U.S. workforce, which numbered nearly 620,000 in 1979.

GM did not directly comment on Trump’s tweet.

“GM’s China operations are not a threat to U.S. jobs,” the company said in a fact sheet, noting that its joint ventures have sent $16 billion in equity income to GM since 2010 and that it has invested $23 billion in U.S. operations since 2009.

GM’s U.S. hourly workforce has fallen by about 4,000 jobs since the end of 2018 to about where it was a decade ago.

Trump’s ire with GM comes as contract talks with the United Auto Workers union with the Detroit Three automakers intensify ahead of a Sept. 14 deadline. Trump has previously attacked GM for building vehicles in Mexico and for ending production at plants in Michigan, Ohio and Maryland and threatened to cut GM subsidies in retaliation.

GM’s decision to close four plants in the United States is a central issue in the contract talks.

Trump has made boosting auto jobs a key priority and has often attacked automakers on Twitter for not doing enough to boost U.S. employment. His 2020 re-election bid will hinge on holding key industrial battleground states like Wisconsin, Pennsylvania and Michigan that narrowly voted for him in 2016.

China is the world’s largest auto market, and government policy favors automakers assembling vehicles there, and not importing them from overseas.

In response to Trump’s latest tariffs, China said last week it will reinstitute 25% tariffs on U.S.-made vehicles. The U.S. is imposing 15% tariffs on more than $125 billion in Chinese goods starting Sunday.

GM sold 3.6 million vehicles in China last year accounting for 43% of its worldwide sales. GM booked $2 billion in equity income from its China operations last year.

GM imports a small number of vehicles from China. In June, the Trump administration rejected a request from GM to exempt its Chinese-made Buick Envision from a 25% U.S. tariff on sport utility vehicle models.

The midsize SUV has become a target for U.S. critics of Chinese-made goods, including leaders of the UAW members in key political swing states such as Michigan and Ohio.