To me, Krugman isn't stupid. He knows the argument is petty and trite, but that's not what he's trying to do here.
He's playing uber partisan. The idea is to use this as a whip, to fan the flames of anger in the people on his side, the Leftists and Progressives that hate all conservatives, Christians and Republicans.
It's really is that simple.
The problem is, facts have caught up with Krugman, and there has been a outpouring of articles like this and more that have proved him not only factually wrong, but extremely biased and partisan to the point of making his point, albeit in the opposite way he intended, for him!
Krugman, it is YOUR rhetoric that is shameful!
I would like to see a tide of outraged Americans denouncing these left wing partisan and completely biased prejudiced around the bend attacks and not only denounce them but denounce them in the most forceful way possible. The only reason for these leftwing commies to engage in such things is because America rejected their agenda in the midterm elections forcefully and now the leftwing commies refuse to accept that result and so these leftwing commies feel the need to incite the nutjob members of their party and incite them to violence.