The campaign of the GOP nominee has now gone full meta by lying about lying.
It absolutely IS lying to falsely represent that you know a matter to be true, even if you don’t know whether or not the matter is actually true. This is referred to in legal authority as “reckless disregard for the truth.”
For example, California Penal Code section 125, which has been in effect since 1872, states that “An unqualified statement of that which one does not know to be true is equivalent to a statement of that which one knows to be false.” Thus under California law such statements can be prosecuted as perjury. I am sure most or all other jurisdictions have similar laws.
The U.S. Supreme Court has repeatedly held that statements made with reckless disregard for their truth, as with knowingly false statements, are not entitled to constitutional protection under the First Amendment. For example, see Garrison v. Louisiana or N.Y. Times v. Sullivan.
The BBS software is not allowing me to include more links, but the Supreme Court cases are readily available on the Google for the truly avid readers and/or insomniacs in search of a cure.