"Warren was useful in the Senate before Trump. She is essential in the White House after Trump,” said Gary Wills, one of my favorite history writers.
Like you, I have high regard for Garry Wills.
However, there are only three online references to the quotation you cite.
One of those three links suggests Wills only communicated his support for Warren in a personal "newsletter."
Here are three blog posts about Garry Wills:
2013:
Pax
Alan