So, your choice is to stick with King and lose the seat to a Dem or vote for his challenger and get someone who might not always vote in lockstep with the Dems. Your call.
We do not know that Kings opponent is any more electable than King. This is a heavily Republican district and electability should not be an issue but the last election, out of state money poured into the district spreading the usual lies about King and he survived, only winning by three percentage points. Where is the evidence that his opponent will have a better fate? Democrats are not going to play nice because they got rid of King. The only polling I know of is the Sec of State electronic state fair poll that showed King with twice as many votes as his closest opponent. King also way outpolled the Democrat. There was a county party poll which was decidedly won by the opponent, so who knows.