As the conservative polemicist Pat Buchanan prepared last fall for the release of his book “Suicide of a Superpower: Will America Survive to 2025?,” some friends who worked with him at MSNBC were worried. The book, they told him, would provoke controversy and threaten his professional well-being.

Undeterred, Mr. Buchanan began his book tour, but his friends were right. He stopped being asked to appear on shows on MSNBC, the cable news channel where he had been employed for nearly 10 years. On Feb. 16, the channel said in a brief statement: “We’ve parted ways with Pat Buchanan. We wish him well.”...

Timothy Stanley, a British historian whose biography of Mr. Buchanan, “The Crusader,” was released two weeks ago, said he thought the departure was a “marketing decision” by MSNBC, a unit of Comcast. “They took an opportunity to get rid of someone who was doing damage to their liberal brand.”...

Mr. Stanley said that “Buchanan is very clever and careful at not saying whether the end of white America is a good or bad thing.”...