It’s only a nonbinding preliminary vote, but union members who operate and maintain CTA trains overwhelmingly decided they’d be willing to go on strike if an 18-month stalemate in contract negotiations drags on.

The vote “came up with a 98 percent ‘Yes’ on locking up the city if we wind up going in that direction,” Kenneth Franklin, president of Amalgamated Transit Union Local 308, said at a news conference held Monday at City Hall.

“The vote was a litmus test, a survey of the membership’s pulse,” he said, adding that no timeline has been placed on a second round of voting.

About 1,000 of the union’s 3,000 members took part in the June 29 preliminary strike vote, according to the union.

“We do not want to cause the city a heart attack, but our patience is worn out, our anger is growing and we demand that they come to the table and meet our very fair demands,” Franklin said.

A statement from the Chicago Transit Authority expressed bewilderment: “The Union’s contract with the CTA prohibits its members from striking for any reason. CTA service will continue as scheduled.”

The union disputes this.

“My lawyers have not found that. My professors that I work with that teach law have not discovered that. The only thing we found in state law is a five-day notice to an employer,” Franklin said.

The CTA statement continued: “If Local 308 proceeds with an illegal strike in the future, CTA will take all actions necessary to maintain service for our customers.”

Negotiation sticking points include wages, increased cost of medical coverage and the CTA’s ability to scale back full-time workers in favor of part-time labor.

Announcement of the voting results came a week after an op-ed authored by Mayor Rahm Emanuel appeared in the New York Times boasting about the CTA — but “not once did he mention the union,” Franklin said.

Franklin said union members take pride in being part of the “economic engine of Chicago” and prefer not to strike, but “I’ve received nothing but disrespect at the [negotiating] table.”