Ok so Reps are saying, without investigating anything, that Brett Cavanaugh did not sexually assault anyone in high school.
But they also don’t want the FBI to investigate. Why not? If they are convinced that he didnt do this, what could be the harm in an investigation? 
If he is innocent and the claims are false, the investigation will prove them right. You’d think they would welcome an investigation that could clear the name of the person they support for SCOTUS.
The answer is two-fold:
1. They are in a hurry to get their nominee confirmed before the midterms and they know that an investigation would delay this.
2. An investigation might prove the allegations true and A) They look like tools for supporting him. B) It might mean he can’t be confirmed. C) It will force them took address an issue that would rather just sweep under the rug.
And so…in conclusion,
1. Reps accused Dems of being aware of the accusation but holding onto it until now prompting them to use that as an excuse not to investigate
2. Reps wanted to ignore the accusation and push through the nomination.  They felt like an investigation would take too long and would push things past midterms when more Dems would be in Congress to weigh in. This could put this nomination in jeopardy.
3. Reps apparently knew about the second accuser but were hoping it would stay under wraps as it puts an even more severe wrinkle in the nomination which was supposed to be a slam dunk.
So to sum up: Reps are doing the very same thing they are accusing the Dems of, which THEY say would delay the confirmation.