Well, this has elicited all the obvious outrage. (Jay Carney: "But to go so far as to make the safety of the President’s children the subject of an attack ad is repugnant and cowardly.")
Obviously the President needs SS protection for his daughters because if they were kidnapped there would be some serious national security problems. So, yes, the ad is completely unreasonable.
The thing is - this ad isn't aimed at reasonable people.
The NRA's tactic here (must?) seems to be to rile up its most vocal supporters. The people out there who listen to Rush and think the government is coming for their freedoms and blahblahblah - they'll like this ad. This keeps them engaged, and makes them write more letters.
After all, the NRA is savvy enough to see what the political situation is in Washington. They only need to hold the line in one branch of Congress (i.e. with the House Republicans), and then all the moderate red-state Senate Democrats are not going to want to push hard for new gun legislation. Think Heidi Heitkamp, Joe Manchin, even Harry Reid. Why should they empower their opposition back home if the House will never vote on what they pass?
In the end, the NRA only wins the gun violence debate if they maintain stasis. To do so, they don't need to win people to their side - they simply must maintain the intensity of their already-devoted constituency. Or at least that's how they think.
And given what the state of Congress has been, this isn't a dumb strategy.