Santorum is so divisive, that they see him as the easiest to beat. And believe me, after Santorum lets his arrogant manner take over once he thinks he can't be beat, he will open his canyon sized mouth and stick both feet into it.
Which is exactly how the Democrats see him and why he will be so easy for them to beat.
I am glad you pointed that out. It’s mainly among conservatives that Romney is considered the weakest candidate. Some liberals agree, but there is no consensus among liberals and many think Romney with his money and organization and good V Obama poll numbers now will be the hardest to beat. They are split on this. And some think Romney winning wouldn’t be so bad for liberalism too. No consensus at all.
But without a doubt, there’s a huge number of libs who are convinced Santorum would be by far the easiest. In fact, they hadn’t even thought that kind of luck was possible til recently.