Ezra Klein, an analyst, columnist and television commentator who runs The Washington Post’s Wonkblog, is making plans to leave the newspaper after failing to win support for a new website he wanted to create within the company, according to four people with knowledge of the negotiations.

Mr. Klein, who quickly ascended into the ranks of the Washington media establishment with a multiplatform blend of policy nuance and number-crunching on Wonkblog, approached Katharine Weymouth, the Post’s publisher, in recent weeks, the people said.

After consultation with the newspaper’s editor, Marty Baron, according to one of the people, he put forward a proposal with detailed revenue projections to build a new website dedicated to explanatory journalism on a wide range of topics beyond political policy. It would have been affiliated with The Post, the person said, but would have been a separate enterprise. The investment he sought, the person said, was in eight figures.

Ms. Weymouth and the paper’s owner, Jeff Bezos, declined to support the project. Since then, Mr. Klein has had discussions with several potential investors and venture capitalists in an effort to start the website himself, said those with knowledge of his plans, who insisted on anonymity in discussing them.