Dershowitz argues that if Mr. Smith loses the case he's bring against Mr. Trump he will himself be vulnerable to being charged with conspiring to punish Mr. Trump for exercising a right, free speech, guaranteed by the Constitution.
If Trump is found to have had a legal right to say the things he said on January 6th, whatever one thinks of the wisdom of his speech, then Smith will be ipso facto guilty of conspiring to "injure" Trump by sending him to jail for it.
Here's an excerpt from the linked article:
Let me explain to you why [Smith is taking a huge risk],” Dershowitz said. “The [Constitution is violated when] two or more persons conspire to injure and deny somebody the free exercise or enjoyment of any right or privilege secured him by the constitution.Dershowitz continues his criticism of Smith's indictment:
What if a court ultimately rules that Donald Trump had a right under the First Amendment to make his Jan. 6 speech and to do what he did?
Then Jack Smith will have conspired to deny him of that right. That’s how serious this is.”
“Jack Smith … deliberately, willfully and maliciously leaves out the words that President Trump spoke on Jan. 6 in his terrible speech. which I disagree with, but what he said was, ‘I want you to assemble peacefully and patriotically,’”The potential violation of Mr. Trump's First Amendment rights isn't the only danger for Smith:
Dershowitz added, “Jack [Smith] leaves that out. That is a lie, a lie, an omission lie and if you’re going to indict somebody for telling lies, don’t tell lies in the indictment.
If you’re going to indict somebody for denying people their constitutional rights, don’t deny them their constitutional rights by indicting them for free speech. That’s how hypocritical this is.”
Dershowitz earlier said that Smith’s indictment targeted Trump’s Sixth Amendment right to counsel, by naming attorneys who represented Trump as unindicted co-conspirators....So, Smith better win his case against Trump or else he may find himself prosecuted for serious crimes against Trump and his attorneys. This hazard certainly makes his allegations against Trump an example of grabbing hold of a tiger by its tail.
“So ... this is a very, very dangerous indictment, dangerous to the First Amendment and also dangerous to the Sixth Amendment because it directly goes after Trump’s lawyers, names them as unindicted co-conspirators without giving their names, but basically says they’re criminals for giving him advice on how to challenge the election.”
You can see the video of Dershowitz making his argument at the link. The video makes it pretty clear that Jack Smith's case is exceedingly weak.