the Republican vaccination strategy
The Republicans have been saying for years that Hillary is corrupt, in spite of continuous investigations for years that have turned up nothing. They accuse her of lying, even though lies have never been documented. And now Trump is saying that Hillary is probably cheating on Bill.
Low-information voters will hear this, see the headlines that both candidates are accused of the same thing, and dismiss both charges as unimportant. this vaccinates Trump against losing votes from his base because of these scandals.
Accusing the opposition of doing what he himself is doing is unfortunately an effective Karl Rove strategy. In the 2004 election, they protected Bush against the charge of having a questionable military record by questioning Kerry's.
One thing that can be concluded from Trump's latest charge: He is probably cheating on his current wife. He has done it often enough before, why should he be any different now?