Pernicious:
Having a harmful effect, especially in a gradual or subtle way.
‘the pernicious influences of the mass media’ [Oxford English Dictionaries]
Noted in GOP Senator Jeff Flake’s speech to the Senate today, of which a transcript may be found here. The relevant passage:
But many untruths are not at all trivial — such as the seminal untruth of the president’s political career – the oft-repeated conspiracy about the birthplace of President Obama. Also not trivial are the equally pernicious fantasies about rigged elections and massive voter fraud, which are as destructive as they are inaccurate — to the effort to undermine confidence in the federal courts, federal law enforcement, the intelligence community and the free press, to perhaps the most vexing untruth of all — the supposed “hoax” at the heart of special counsel Robert Mueller’s Russia investigation.
