People close to the case say prosecutors are likely to bring charges based on a rarely used provision of a fraud statute, under which they would argue that Mr. Christie’s associates used the bridge, or the Port Authority of New York and New Jersey, which runs it, for a purpose other than its intended one. In the case of the bridge, the closings were apparently meant to punish Mayor Mark Sokolich of Fort Lee, a Democrat, after he declined to endorse the governor’s re-election bid in 2013.