The funniest part is that Trump could have investigated Biden in a totally legal way.

Trump could have asked the DOJ to investigate Biden with respect to corruption in Ukraine.
Barr's DOJ would then have made up some rationale for why this investigation is warranted and would have reached out to the State Department.
The State Department would have reached out to ukrainian diplomats.
The ukrainian diplomats would have reached out to ukrainian prosecutors.
And the result would have been a joint investigation coordinated between the DOJ and ukrainian prosecutors.

Everything perfectly legal, perfectly transparent and with an unimpeachable paper-trail as innocent as freshly fallen snow.



And what did Trump do?
He personally asked an ukrainian politician to tell the ukrainian prosecutors to make the investigation happen.
He sent his private attorney to investigate, not a US government official.
And instead of coordinating with ukrainian law-enforcement, this private attorney acted abroad all by himself.



Why?
Trump didn't want a transparent investigation.
He didn't want an investigation with a paper-trail.




So, when Rudy Giuliani asks on Fox News whether Joe Biden is above the law purely by merit of being a candidate, the answer is No.
It's perfectly fine to investigate Joe Biden... as long as you do it by the book.