Kudlow had worked as a White House budget aide in the Reagan administration, and had also been an informal advisor to the 2016 Trump campaign.
When he was a CNBC contributor before taking the White House job, Kudlow argued the metals tariffs Trump had announced would harm consumers.
Mr. Trump hired him in April to replace Gary Cohn as the head of the National Economic Council, reportedly because he enjoyed Mr. Kudlow's combative television appearances.
"Here's the thing. He really kind of stabbed us in the back", Kudlow said on CNN Sunday. The associates spoke on the condition of anonymity due to sensitives of Kudlow's health status.
Before joining the White House, Kudlow vocally opposed the protectionist trade agenda that Trump promoted, but he has become a forceful advocate for the president's policies since joining the administration.
For five years before his White House appointment in March, Kudlow hosted a show on CNBC on business and politics, and he's worked at the Federal Reserve Bank of NY.
But since joining the Trump administration, he has faithfully executed his boss's agenda - arguing for instance that the European Union must make "concessions" if it wishes to be exempt from tariffs, and signaling the U.S. is prepared to walk away from two-decade-old North American Free Trade Agreement.
Kudlow, who is a wry and upbeat speaker, has adopted some of Trump's hard-line rhetoric while working inside the West Wing. As recently as Sunday, he had appeared on TV defending the president's attacks on Canadian Prime Minister Justin Trudeau, saying that Trudeau had committed a "betrayal". His recovery was a great source of pride for him and for his friends, contributing to his upbeat manner.