Read between the lines here in this Lindsey Graham statement:

[dpibel]

2. I believe it's a mixed-motives argument

I understood the Trump team to be developing the argument that, so long as he had some legitimate reason for withholding the aid, it doesn't matter that he may also may have had an illegitimate reason.

It's a defense that (depending on the law of the jurisdiction) can work in discrimination lawsuits: You have to prove that discrimination was the sole reason for termination; an employer can escape liability if there was some legitimate reason for termination.

That's the best sense I can make of this line of argument.