Donald Trump's references to Bill Clinton's affairs risks having the tables turned on him. Trump had a very open affair with model Marla Maples in the 1980s and 1990s that led to his divorce from his first wife, Ivanka Trump. He did not hide the affair. Indeed, he seemed proud of the publicity it created.
Trump is a known hypocrite. He points out Bill Clinton's infidelity, yet it is well known he has cheated on his now ex-wife. This has been his campaign from the very beginning, and it is unsurprising that this is how he is trying to salvage his campaign now. He can't talk politics so he talks about the Clinton's sex lives.
If Trump is referencing Bill Clinton's affairs in a disparaging way, he had better expect his rivals to see what they can dig up about him and his personal life. The asme applies to any accusation he makes against Hillary Clinton - he needs to be sure he is not guilty of the same or similar 'crimes' if he is to retain credibility with the American voters.
Although, Bill Clinton's public scandals while in the Oval Office have made him somewhat of a pariah, Trump's own multiple affairs have made him subject to the same criticisms. While Clinton had multiple affairs while in office, he remains married to the same woman. Trump however, left his wives for the women he had affairs with, which may leave him open to greater scrutiny.