What was the exact quote? The article doesn't mention it. I assume it was in support of the HK protestors.
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The Chinese are correct to be afraid, they have spent the last half century cleansing the concept of freedom from their society and they are terrified of what comes next.
People are going to start dying publicly in HK very soon, and the rest of the world will have to decide which side of history they want to stand on.
The American's needed allies to succeed in their revolution against a much more powerful nation, HK will need allies too.
His "trade war" already puts him in an adversarial stance towards from the perspective of his base.
If it can someone be framed as a choice between the freedoms that Americans have died for and bending over to a totalitarian state I think it would be possible to sway conservatives into opposing these policies.
Having China and the US grandstanding will make it a wedge issue and decrease the chances of compromise and a peaceful solution. At some point the parts need to sit down at the table and find a solution they can all present as a win.