RITIKA SINGH and BENJAMIN WITTES – Political parties in the United States, like a spatting couple in a bad marriage, have been fighting over the law of counterterrorism for more than a decade. And like the spatting couple, they have developed an almost rote script for their fight. The script has a logic of its own. It is a comfortable one for both spousesâ€”and the fight is soothing in its own way. Republicans and Democrats alike wrap up some portion of their partyâ€™s identity and self-image in the conflict over national-security policy. The fight gives each side the impressionâ€”and the confidenceâ€”that the other endangers America. And it gives each side something to tell voters about why they should vote one way rather than another.