How does Harry Frankfurt's voting-booth example challenge the ability-to-do-otherwise principle?

Harry Frankfurt’s "voting-booth" example challenges the principle of alternate possibilities—the idea that moral responsibility requires the ability to do otherwise—by demonstrating that an agent can be morally responsible for an action even when they lack the power to choose a different path.1

In this thought experiment, an agent named John intends to vote for a specific candidate. Unbeknownst to him, a device has been implanted in his brain that would force him to vote for that candidate if he showed any inclination to choose otherwise. However, John decides to vote for the candidate on his own, and the device never activates.1

Because John acted according to his own volition without the device's interference, he is intuitively considered morally responsible for his choice. Yet, because the device would have intervened had he wavered, he lacked the actual ability to do otherwise. This "Frankfurt-style" counterexample suggests that the ability to do otherwise is not a necessary condition for free agency or moral responsibility, shifting the focus of freedom toward the internal identification with one's own desires.1