I think they would have against Obama... but NOT Hillary. Obama had a relatively clean slate before he took his first term. He didn't do anything astronomically good or bad (he didn't do much at all really). So when McCain/Romney ran against him, they got a beat down. Now, I know Romney is a Mormon, but I think he is a "good" man comparatively speaking to Obama/Hillary/Trump/Bush/etc...
Romney against Hillary, I think Romney would have won in this 2016 election. Of course, this is just my speculation/opinion. Who knows what really would have happened. I believe Romney would have won because of his abilities but more importantly, Clinton is so disgustingly crooked.