DEARBORN, Mich. — Leaders of the United Automobile Workers union said on Wednesday that a slight majority of workers at Ford Motor Company have so far voted against a proposed new labor agreement.

The tight results mean the deciding votes are in the hands of workers at three large assembly plants in Michigan and Illinois that begin voting on Thursday.

U.A.W. officials are urging those workers to support the four-year tentative agreement, and avoid a defeat that would force the union to go back to the bargaining table with Ford.

Jimmy Settles, the union’s lead negotiator with Ford, said at a news briefing on Wednesday that about 75 percent of the company’s 52,000 factory workers had voted so far. Of that number, about 52 percent voted against the agreement, he said.