Nathan Bomey

USA TODAY

The United Auto Workers remains interested in unionizing the Tesla Motors assembly plant in California, the union's president told reporters Thursday.

The 5.3 million-square-foot Fremont, Calif. factory is the only U.S. assembly plant owned by an American automaker that is not represented by a union. Workers for foreign automakers with U.S. plants generally do not have union representation.

Although the Detroit-based UAW has previously expressed in organizing the Tesla plant, the stakes are considerably higher after the California automaker said Wednesday that it plans to manufacture 500,000 of its electric vehicles annually at the factory in 2018 — about 10 times its 2015 level. The announcement moved up its timetable for 500,000 by two years.

"We’re watching that very closely," UAW President Dennis Williams told reporters in speaking about Tesla. "We just believe workers ought to have a voice in the workplace, and they ought to have collective bargaining rights."

Tesla did not immediately respond to a request seeking comment.

Williams said the union is "not approaching this in an adversarial way."

He said the union long respected Tesla's status as a start-up company. But the automaker's production ambitions would make Tesla the ninth largest seller of new vehicles in the U.S. — just behind Volkswagen but ahead of BMW and Mercedes-Benz, as measured by 2018 figures.

Tesla announced Wednesday that it would raise up to $1.7 billion through a stock offering to fund the production expansion of its Model 3 electric sedan, its first mass-market model. Tesla plans to start sales of the Model 3 in late 2017, joining the two luxury models it sells now.

Williams said he has met with Tesla CEO Elon Musk, though he did not specify the timing or say whether they discussed the possibility of a unionized workforce. Williams described Musk as "creative" and a "very unique individual."

He did express a degree of skepticism regarding Tesla's production plans. "I don’t think they’ve ever met their mark yet on production," Williams said.

Also Thursday, Williams was peppered with questions regarding the UAW's plans for endorsing a candidate in the presidential election. The union is deciding between Democratic contenders Hillary Clinton and Bernie Sanders but has not rendered a decsion yet, he said.

He blasted presumptive Republican nominee Donald Trump for comments suggesting that plants should be shifted to low-cost towns in the South.

"We ruled him out real quick," Williams said.

He acknowledged, though, that 28% of UAW workers who responded to a survey about the election said they backed Trump.

Follow USA TODAY reporter Nathan Bomey on Twitter @NathanBomey.