[Reporter] The sounds and images stunned America.
Not since the war of 1812 had anyone stormed the U.S. Capitol building.
Surely there would be a price to pay for whoever caused the rioters to do this.
Say Democrats, the culprit is clear.
The rioters were incited by Donald Trump who egged them on.
We will never give up, we will never concede.
It doesn't happen.
[Reporter] In that speech, at that rally in his waning days as president.
What followed soon after the world well knows.
[Pelosi] Resolution is adopted.
[Reporter] Within days, Trump was impeached in the House of Representatives for inciting an insurrection.
[Pelosi] Donald Trump is a clear and present danger to our country.
[Reporter] It is now up to the U.S. senate to convict him
and perhaps disqualify Donald Trump from ever again running for office.
At issue in his trial, a handful of key questions.
All leading to the big one.
What caused the rioters to riot?
Did Trump intend for his speech to be taken as a call for violence?
And is it reasonable to believe that his words would have that effect?
Democrats underline before you get to the rioting
it's important to understand that when Trump talks, Trump voters listen hard
and so context matters.
When Trump falsely claimed the election was stolen
that message resonated with Trump voters
and Trump said it again and again.
This is a fraud on the American public.
This is an embarrassment to our country.
They're trying to rig an election
and we can't let that happen.
[Reporter] His lawyers argued the same thing in court after court.
But even though in every meaningful case they lost
Trump kept at it for all who would listen.
Emotions ran high.
This rally was two days before the riot.
[Trump] And they're not talking this White House, we're going to fight like hell I'll tell you right now.
[Reporter] Then on January 6th outside the White House that final rally.
On stage, Trump lawyer Rudy Giuliani.
Let's have trial by combat.
[Reporter] And into that atmosphere Trump himself.
The crowd was supercharged.
[Trump] You have to show strength and you have to be strong.
[Reporter] He spoke of being peaceful
but, say Democrats, the key words were these:
[Trump] We're going to walk down Pennsylvania Avenue, I love Pennsylvania Avenue
and we're going to the Capitol.
[Reporter] And then these:
[Trump] And we fight, we fight like hell
and if you don't fight like hell you're not going to have a country anymore.
[Reporter] It was soon after that the Trump supporters marched on the Capitol
with their MAGA hats and their Trump flags as if to signal this was for him.
[Man] Oh, there we go.
All of which brings us back to the matter of incitement.
Is Trump accountable for what happened?
Lawyers and lawmakers will now gather in that building to weigh Trump's words and the law.
[Woman] He said go to the Capitol, go to the Capitol
and show them what you're angry about.
[Reporter] Michigan law professor and free speech expert Nancy Costello
underlines U.S. law dictates much must be proven.
[Trump] All of us here today do not want to see our election victory stolen.
[Reporter] The right to speak freely in America is protected by the U.S. constitution
but there are limits.
Among them, you cannot incite a crowd to riot
but to be convicted you must prove intent.
What did Trump mean when he said fight like hell?
That's exactly what his attorneys are arguing.
[Reporter] Costello predicts his lawyers will parse those words carefully.
They're saying that what he meant by that is that you have to keep pursuing
if you want to make sure that you have a legitimate election now and in the future.
That he was not saying 'hey, go down to the Capitol, break down the doors'
but he didn't say any of that and so therefore is it incitement speech?
The honesty of our elections and the integrity of our glorious Republic.
[Reporter] Indeed Trump's defense is that in that speech January 6
he was simply defending election integrity.
And of course it will come down to what did he believe was true.
[Reporter]. But as well, says free speech expert Gene Polisinski
Democrats will argue that as president, Trump should have known
some in that crowd came prepared.
That there was already this warning from apparently the FBI
so you can say he could not have conceived it.
That's an argument.
But on the other hand what was his responsibility as President of the United States
to know the potential for that on that day?
Certainly in our country we knew how passionate this argument was around the country.
[Reporter] All of that and more now goes to the test.
Including by the way whether as an ex-president it's now too late to try Trump anyway.
Still, for so many it all boils down to the words he used in that speech on that day.
Did he incite insurrection?
Should he pay a price for what happened
or will Donald Trump walk away again?
Paul Hunter, CBC News, Washington