One party welcomes investigation and has requested it,... the other party does not want any investigation...
Usually,... if you're accused of something bad, you want it to be proven a lie...
Unless.....

Pretty simple logic
I know,... they don't want the nomination stalled,... but wouldn't a innocent person be saying, "Even if I'm confirmed, I want the FBI to clear my name afterward"...
He's not saying that....
He's just saying he's a really good guy... so let's not investigate...
Mmm Hmmm?....