IIRC, they endorsed Dingy Harry a few years ago.
For all the media cacophony that links the NRA to "conservative Republicans," the organization is one of the most effective lobbying groups in Washington because it is politically non-partisan.
The NRA has a long-standing policy of endorsing Second Amendment supporters regardless of political affiliation. I believe they even have a standing policy of supporting an incumbent over a challenger if they are both seen as advocates of the right to keep and bear arms. In other words, the incumbent who has voted accordingly is preferred over a challenger with no track record.
This is why the NRA has been supporting Harry Reid for some time. This @sshole has come out pushing for gun control right now only because he's retiring and won't be running again.