MANILA — Since taking office just over three years ago, President Rodrigo Duterte has not only overseen a murderous campaign on drug users and sellers. He has also unleashed a brazen assault on the country’s democratic institutions — at times, using his so-called war on drugs as a pretense for going after his political adversaries and dissenters.

I should know: I’m one of its victims. I am writing this essay from a prison cell in Camp Crame, the national Police Headquarters in Manila. I have spent the past two years here, after being arrested on fabricated drug-trafficking charges. But the only crime I committed was to use my platform as a senator to oppose the brutality of this administration’s campaign against drugs. And I hardly am the only target.

Mr. Duterte’s government has orchestrated the removal of a Supreme Court chief justice and harassed and sidelined Vice President Leni Robredo (she belongs to a different party). Independent media houses have been bullied with bogus criminal charges; one was effectively pressured into being sold to Duterte allies. The president has publicly threatened human rights activists and others with death — never mind that he or his aides often then downplay his statements as lighthearted banter.

But most worrisome, perhaps, is the administration’s effort to cow what little remains of the formal political opposition, often through politicized criminal cases.