They are single issue. I suppose Reid has stood with them in the past, so they are obliging.
On top of that the NRA probably finds it is good policy to support at least a couple of DIMS, so they can not be accused of being single party and loose some legal status of some sort.
That may be, but I'm sure they could find a conservative Zell Miller type Democrat. Not a real Democrat, I know, but if the "inspectors" are sitting there claiming all that matters is the letter after the name, then they're stuck with that mode of analysis, too.