York's Dinan Ackman's Latest Investor Enemy

Pershing Square Capital Management's William Ackman has a new peer-adversary.

Already sparring with longtime nemesis Carl Icahn over nutritional supplements company Herbalife—which Ackman calls a "pyramid scheme" and Icahn an opportunity—Ackman now finds himself opposing York Capital Management's James Dinan on retailer JC Penney Corp.

In an inversion of the Herbalife situation, Ackman is extremely bullish on Penney, while Dinan said last week that York is shorting the company's debt. He also gently criticized Ackman and other activists, noting that activism "works great when the tide is rising." But he said that Ackman faces trouble, given Vornado Realty Trust's recent sale of half of its Penney stake, Reuters reports.

Ackman has been investing in Penney since 2010. The retailer's stock price has been falling, and its CEO, former Apple Inc. executive Ron Johnson, was recently forced to abandon his signature program at Penney, an end to sales.

Pershing Square is currently nursing a loss on its Penney holdings.