My guess is the Dems want us to destroy the missile while it is still on the launch pad because they don't want us to use the missile defense system. if we destroy the NK missile in the air using missile defense then that would prove the system works and in turn make the Dems look bad for opposing it in the first place.
Imaging every incombent republican up for re-election using this against their dem opponent, in a close race something like this used effectively might just do it for the incombent.