At his rally last night, Trump said that the House of Representative's Impeachment Investigation was "unconstitutional"?

Not true on any level.

Trump is in a panic

Aside from his estranged relationship with the truth in general, being semi-literate and having a grade school level comprehension of and/or interest in government, it's unlikely he has ever looked at, let alone read, the Constitution. 

That said, while he doesn't know or care, saying the House Impeachment Process is 'unconstitutional" and as a result will "destroy the American justice system" is particularly Trumpian Flim-Flam Man talk.  Because the 2 are entirely separate of one another with Impeachment being the only option since the Justice System has no legal course of action(s); indict, charge, prosecute etc. against a sitting POTUS.... 

Subsequently, I am going to say it is not a true statement.     

trump rarely tells the truth.  He didn't this time, either.

If it’s illegal to impeach a fascist psychotic president the system is broken.

In these cases, let the egomaniac run his mouth and boast his stupidity to his stupid moronic supporters.  Don’t take what he says with a grain of salt.  The people will win in the end and this horrible human being and his racist bootlickers will lose.

I don't think that donnie has a good enough grasp of the Constitution to make that call.

Of course not, Trump has no credibility in this.

No.  Of course not.  And no educated person believes Trump's statement either.  The "impeachment investigation" is right in our constitution.  Trump = 14,000+ verified lies.  No one believes a thing Trump or anyone else in his regime says.

I believe that he said it.  It doesn't make him right.  He's not an expert on constitutional law.

The Lefties are abusing a Constitutional process toward an unconstitutional  end .

It's sort of like saying that breaking  laws is part of the legal process .

Malicious prosecution and frivolous lawsuits are part of the legal process too .