McGuire is laying out an ultimatum for Deckers, saying that if the company's strategic review process doesn't lead to a sale, he'll seek to replace the entire board.
"Should Deckers' strategic review process not culminate in a sale of the company at an attractive value to all stockholders, we will be prepared to seek significant board change at the company's next annual meeting by nominating a slate of director candidates to replace the entire board," McGuire, the managing partner of Marcato Capital Management, wrote in a letter, obtained exclusively by CNBC, to the board.
McGuire is turning up the heat after running a proxy contest at Buffalo Wild Wings – obtaining three of the four director spots he was seeking and the retirement of CEO Sally Smith.
Marcato's fund is up 8 percent year today, a person familiar with the returns told CNBC.
Several months after McGuire disclosed a 6 percent stake in Deckers, the company said it had hired Moelis to initiate a review of strategic alternatives, which often means a sale.
But in the letter reviewed today, McGuire says that process has lacked transparency. He adds that the current board and management team has a history of quote "missteps" that cause him to be concerned whether a sale will result in the maximum value for shareholders.
He points out that the Chairman Angel Martinez recently started a campaign to become the mayor of Santa Barbara, which may distract him from the strategic review process.
Deckers shares closed up more than 1.1 percent on Tuesday. In a statement, Deckers said: "We appreciate the views of our stockholders. As previously announced on April 25, 2017, our Board of Directors is reviewing a broad range of strategic alternatives to enhance stockholder value. As always, our Board of Directors will continue to take actions that are in the best interests of the Company and all stockholders."