Why are you sure he would have? I'm not.
From a pragmatic standpoint, what he says makes a huge difference. If he can't politic his way out of a question, how would he be any kind of effective in office? Even if he was completely pro 2a that is only one thing. There are many things I didn't like about him. Please don't take my post as an endorsement for Donnelly, he didn't get my vote either.
From the sounds of it, Mourdock never had your vote anyway, and not really for pragmatic reasons. Mourdock answered the question the way a politician from the 90s or early 2000s would answer the question, back when Evangelicals dominated the Republican Party. Back then his answer would have had been the right play politically. His answer shows he's out of touch with the social evolution of the electorate, much like most people in the Republican Party.
But it all comes down to this. Donnelly will surely vote "yea" on gun control when it comes up. Even as an agnostic, as repulsive as it is, I'd rather have this evangelical social conservative, who ran on a platform of not caving to spineless leaders, in control of THIS vote. Now all we can hope for is that Coats will cancel Donnelly's vote. After his fiscal cliff vote, I'm not sure where his head is on this.