Well, both of us know that lots of political activists have been executed ostensibly not because of their politics but with a charge that they committed a murder or something like that. Like Joe Hill, for instance. Though the state may execute people for explicitly political reasons as in the case of the Rosenbergs, most of the times, the state uses some other charges, as in the case of Mumia. You may make a distinction, but the ruling class and their machinery of repression don't.
More broadly, for American leftists to accept the legitimacy of death penalty for individual murders while the U.S. government may use covert and overt means to murder and otherwise destroy its political enemies at home and abroad seems to me to be the same as being 'in favor of capital punishment in the realm of politics,' as you put it.
Yoshie