Rasmussen is a very reliable poll and there are others which show similarly high negatives for Romney. Mason Dixon in June and Gallup in August had Mitt the most disliked of all the Republican candidates. And this is in spite of the millions he has already spent.
The fact that he alone is as unpopular as Hillary means that he is one of the Republicans she can beat.
BTW the poll is weighted according to the numbers of self identified voters and seems accurate according to what I have read about current party self identification.
I don’t think that’s a very accurate baseline, though - party affiliation doesn’t fluctuate on a whim.
No matter, it’s true that about Romney, although I’ve seen Hillary’s “negatives” much higher than 43.