In an increasingly heated, drawn-out exchange, Ackman and ADP CEO Carlos Rodriguez have been trading barbs as Ackman pushes for seats on the company's board, arguing that his candidates will help the company seize on an opportunity to improve its business model.
"Our new question for ADP is why is it that ADP has lower revenue productivity than all of their competitors?" Ackman asked. (The investor had been referring to ADP's "labor productivity," a Pershing Square representative clarified to CNBC.)
In the Wednesday exclusive interview with Jim Cramer on CNBC's "Mad Money," Ackman said that ADP generates average revenue of $160,000 per employee, while its competitors average $224,000 per employee.
"When you think about ADP, it has enormous scale versus the competitors. So, if anything, they should have more efficiency," Ackman told Cramer.
Ackman mentioned Paychex CEO Marty Mucci's Tuesday appearance on "Mad Money." Twenty-five percent of ADP's business competes directly with Paychex, another payroll processing player that focuses mainly on assisting small- and medium-sized businesses.
Mucci said his company was able to maintain its margins by keeping expenses "out of the business" and using the money to invest in technology. Ackman said he would like to see ADP mimic some of that strategy.
But however sound the suggestions may seem, Ackman acknowledged that activist investing is no easy business. Much of it has to do with "tactics," which may have been why Rodriguez, ADP's CEO, was so vocal about Ackman's strategy when he was in the public eye.
"I think the advisors who advise companies on defending themselves from activists, they say, 'Look, you don't want to show that you're even open to what they have in mind, because if you do, then a lot of new people are going to come into the stock, a lot of more event-driven investors, and they're going to make it inevitable. You're going to lose a proxy contest,'" Ackman said.
The activist investor added that he remembered his early, private conversations with Rodriguez being "all cordial, but when he went on TV, he ... behaved otherwise."
Conflict aside, Ackman told Cramer that ADP is something of a symbol when it comes to his central mission as an activist investor.
"When the founder is around and in the boardroom, you've got a major shareholder there, companies don't lose their way. It's typically after the founder steps off the board, retires, passes away ... [that] the board becomes more professionalized. There's no one in the boardroom that owns a lot of stock in the company. And they get a little complacent," Ackman said. "I think what activism is about is putting major shareholders in boards of directors so even though the founder is gone, there's someone there watching the store."
Ackman added that his firm's $2.3 billion stake in ADP puts him "dollar-for-dollar" with the company's other investors. The proxy battle will come to a head at ADP's annual shareholder meeting in November.
"I think shareholders, really, all they care about is, is there an enormous opportunity to improve this company? Can adding a major shareholder to the board increase that probability? And I think the answer that we're hearing is yes," Ackman said.
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Clarification: This article has been updated to reflect that Ackman had intended to question ADP's "labor productivity," according to a Pershing Square representative.