Yes, well-informed rejection of any dogma of the faith excommunicates. He may have an excuse if he is not well informed of the doctrine, or struggles to accept it, or if it is a lapse into sin and he confesses it, but not if the doctrine says one thing and he does another.
Classic examples are politicians promoting abortion “rights” or couples cohabitating or contracepting.
I should perhaps clarify that this implicit excommunication of a heretic only happens when it is a dogmatic teaching on faith and morals beings not adhered to. Otherwise, it is simply an opinion, perhaps at odds with the prevailing opinion of popes and bishops, but still permissible.
To tell dogmatic teaching from ordinary teaching is not always straightforward. One current example is the teaching against abortion — always, dogmatically, intrinsically wrong — and participation in wars, usually left to the individual’s conscience and circumstance. Much in the Catholic Church’s teaching on social issues of poverty, politics, wars in general, war in Iraq, etc., is not dogmatic. Thge faithful should not ignore these, but they are free to form their own mind about them without falling into heresy.