The bridge manager explained to Mr. Wildstein that the lanes were used to ease congestion from traffic flowing though the town of Fort Lee. They were the result of a deal between a long-ago mayor of the town and a former governor of New Jersey.

Mr. Wildstein quickly recognized that the lanes could be a point of leverage for Mr. Christie, who had been wooing the mayor of Fort Lee to endorse his re-election in 2013.

If the lanes were closed, Mr. Wildstein realized, “traffic would back up in Fort Lee.”

It would be two more years before Mr. Wildstein, a self-described “bad cop” for Mr. Christie at the Port Authority of New York and New Jersey, which runs the bridge, carried out the plot to close two of the three lanes, creating gridlock in the town for four days.

In between, Mr. Wildstein testified, he told Bill Baroni, his boss and Mr. Christie’s top staff appointee at the Port Authority, about the idea. He told Mr. Christie’s deputy chief of staff, Bill Stepien, who was keeping track of endorsements and would go on to run Mr. Christie’s re-election campaign. He told Bridget Anne Kelly, who replaced Mr. Stepien in Mr. Christie’s office, and is now on trial with Mr. Baroni on charges that they conspired to close the lanes to punish the mayor for not endorsing Mr. Christie, and then covered it up.

And when Mr. Wildstein, Ms. Kelly and Mr. Baroni had finally decided to act on the plot and had concocted a story that it had to be done as part of traffic study, Mr. Wildstein testified that he also told a Port Authority commissioner who was seen as “a member of the Christie team,” who “understood” the need to hurt the mayor.