In 1991, the u. s. supreme court overturned the validity of shield laws and ruled that excluding evidence of a prior sexual relationship between the parties violates a defendant's right to a fair trial.?

Not a complete overturn but a revision in the law.

The original shield laws stated the prior relationships of the victim were completely off limits. Defendants argued it was a violation of the 6th amendment rights. 

In 1991, the Supreme Court stated prior relationships could be used as evidence but it had to be presented to the prosecutor and judge prior to trial for review. This gave each a chance to determine validity and relevance. 

Political forces often influence public opinion.