The 78-year-old McCloskey, a maverick known for environmental causes, told The Associated Press he felt compelled to get back into politics because of his concern for the environment and what he sees as a shift in Washington away from traditional Republican values.
He also was prompted by what he described as Pombo's close ties to Washington lobbyist Jack Abramoff, who has pleaded guilty to conspiracy in a growing corruption scandal.
"Winning isn't the issue. The issue is forcing a debate on which way the Republican Party goes," McCloskey said. "This guy Pombo, he wants to privatize the remaining public lands in California and he has the power to do it. He's the chairman of the House Resources Committee. He's up to his neck with Abramoff."
A spokeswoman for Pombo did not immediately return messages seeking comment Saturday.
McCloskey, a lawyer in Redwood City, helped write the Endangered Species Act while he was in congress, and he ran for the Republican presidential nomination against Richard Nixon in 1972 on an anti-Vietnam War platform.
He said Saturday that he spent months trying to find a candidate to run against Pombo, also a Republican.
"I'm going to run against him because nobody else will," McCloskey said. He said he planned to formally announce his candidacy Monday.