The former collegiate swimmer -- who competed for the University of Kentucky, and who also raced against Lia on the NCAA level -- described to U.S. senators what it was like having to change in the same locker room as Thomas ... who's a transgender woman.

She describes seeing Thomas' male genitalia and how jarring it was for her and others, saying nobody ever asked for their consent or input -- they, the NCAA, just let it happen.

In Gaines' eyes, this was a clear violation of their right to privacy, and she went on to pose the question -- what does this trend mean for women's sport? And, more importantly, does anyone care how it impacts cisgender women? By letting Thomas compete with them, she argues the powers that be have proven they don't.

Interestingly, Gaines says she's not advocating for trans athletes to be banned from sports altogether. On the contrary, she says they can and should be athletes -- but it sounds like she's suggesting they have their own classification and not be lumped in with cisgender competitors.

Now, as far as how the lawmakers reacted ... it pretty much fell along party lines. Dems pushed back on her characterization -- and Republicans, by and large, embraced her.