Billionaire Bill Ackman "formally" endorsed President Trump after his attempted assassination in Pennsylvania on Saturday.

More than 19 million people have viewed Ackman's endorsement post since it went out late Saturday night. 

"I came to this decision some time ago as many @X followers have already understood from my supportive posts of Trump and my criticisms of @POTUS Biden," Ackman said. "The reason why I have not yet formally done so is that I want to explain my thinking in detail and address the arguments put forth by others against Trump."

Ackman, the CEO of Pershing Square Capital Management, said it would take a "long-form post" to explain his thinking. He said he did not have the time "nor felt the urgency" to write the post prior to the shooting that took place at a Trump rally that injured the upper part of Trump's ear, left one rally attendee dead and two spectators critically injured.

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Ackman believes the upcoming presidential election is one of the "most consequential" in his lifetime, and says he has spent a few hours recently with Trump. He asked voters to "keep an open mind." He said a more detailed post explaining his position will come in the future.

In January, Ackman said President Biden was not fit to serve, and called on him to step aside during an interview on CNBC's "Squawkbox."

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Billionaire Elon Musk also announced that he "fully supports" Trump shortly after the shots rang out in Butler, Pennsylvania. 

"All of us have an obligation to stand up and be counted," Ackman said in a subsequent X post on Sunday. 

FOX Business' Timothy Nerozzi, Sarah Rumpf-Whitten and Julia Johnson contributed to this report.