I think Republicans are prohibited from filing suits that allege voter fraud.
I've heard there is some sort of agreemnt from past years, but I don't think it prohibits Republicans from discussing voter fraud and presenting the evidence to the public (if such an agreement exists). If any Repubican politician is even dicussing voter fraud very often, then I couldn't name him or her.
It's been folks like John Fund and other journalists who occasionally pop up with a vote fraud story that are about the only ones making the case to the public. Of course, many belive fraud exists including those who work to get voter ID laws passed. It's just not discussed openly very often.