"but I think he should have been more careful in soliciting co-sponsors"
I don't know and am only guessing, but it wouldn't surprise me that the junior senator in that minority party pushed herself onto the sponsorship of the bill while trying to make it look like he solicited her support.
I would guess that your guess is probably correct -- but he shouldn't have made it so easy for her to do that. He's fast become my favorite senator, but then he pulls head-scratchers like this...