The basic premise of the First Amendment is that truth should defeat her opposite number, Falsehood.
Free speech and debate between citizens doesn't apply to accusations made by the government. Those are something altogether different.
In a rule-of-law society, government allegations of criminal activity must be followed by proof and prosecution. If not, government is ruling by innuendo.
Thus, an accusation by President Trump that former President Obama wire tapped Trump Tower, isn't like an accusation leveled by one private citizen against another.
The government's special responsibility has two bases: One is that you can't sue the government for false and defamatory speech. But, if I accused Obama of wire tapping my phone, he could sue me for libel.
On the other hand, if the president makes the same statement, he can't be sued in his official capacity.
The second reason the government has to be careful about making improvable allegations is that its bully pulpit is greater than any other.
True, Obama can defend himself publicly, but he does not have the audience Trump now has.
And an allegation of potentially criminal misconduct made without evidence is itself a form of serious misconduct by the government official who makes it.
When Trump accuses Obama of an act that would have been impeachable and possibly criminal, that's more serious than libel. If it isn't true or provable, it's misconduct by the highest official of the executive branch.
How is such misconduct by an official to be addressed?
The answer is that the constitutional remedy for presidential misconduct is impeachment.