Below are only a few of the heavy-handed tactics federal investigators used to build their case against the McDonnells. See how they compare to how Mrs. Clinton was treated.

Conduct ambush interviews. The first contact between law enforcement and the McDonnells was an ambush interview of the governor’s wife. The agents lied to her about the topic of the meeting, forbade Gov. McDonnell’s staff from attending, and then grilled her on their suspicions about potential public corruption. Statements from that interview later took center stage in the trial of her and her husband.

In Mrs. Clinton’s case, no ambush interviews were conducted, and witnesses were generously accommodated. The FBI and Justice Department even allowed a fact witness and potential target— Cheryl Mills, formerly the State Department’s chief of staff—to simultaneously represent Mrs. Clinton as her counsel.

Immunize only witnesses who can help deliver convictions. One person in Gov. McDonnell’s case got immunity: Jonnie Williams, the prosecution’s star witness. For his testimony, Mr. Williams earned a wealth of blanket immunity—not simply from potential bribery prosecution but also from unrelated crimes he might have committed (including securities and tax fraud). Reluctant witnesses—Gov. McDonnell’s children and friends—were called before a grand jury and forced to testify.