In all fairness, there have been a good many Repubs whose record on gun rights kinda sucked too, though as a matter of statistics you have more anti-gun Dems than Repubs.
I was gravely disappointed in Bush II, when he avowed that he would sign the sad joke misnamed the "Assault Weapons ban" back into law if Congress sent it to him... I was doubly POed that the Repub Senate passed a version of the AWB, but relieved and cheered that the Repub House refused to renew it.
Granted, there are surely a lot of left-wing Dem politicos who would love to see lots of highly restrictive new gun laws passed, BUT they do seem to have caught on to a political fact: gun control is no longer popular and it costs the Dems elections. Not to say they might not try to sneak something in if they think they can get away with it, but it doesn't seem to be front-burner priority anymore.
The majority of politicians, of whatever stripe, are more concerned with preserving their own power and position than they are about any ideological concern. Making this purely a left-right issue probably isn't accurate; for instance you have quite a number of people on DP who self-identify as liberal, but support the 2A and even have a carry permit.
Meanwhile, the OP is really, fundamentally, about a violent nut who finally blew up in a spectacular fashion... and **** like that happens, I don't know that it is really so much a political football as it is just a commentary on human nature.