TRENTON, N.J. -- First it was the leader of a major U.S. pharmaceutical, then the CEO of an athletic gear company, and before the day had ended, the chief executive of a US$170 billion tech giant. Three of the nation's top executives resigned from a federal panel created years ago to advise the U.S. president.

Now, others are pushing for more executives to refuse to serve U.S. President Donald Trump after what many believe to be an inadequate response to a rally of white supremacists in Charlottesville, Virginia, that left one dead and dozens injured.

Announcing his resignation Monday, Merck CEO Kenneth Frazier cited the president's failure to explicitly rebuke the white nationalists. He wrote on Twitter that ""

The response from the president was swift, throwing a jab at Frazier, a highly respected executive and one of only four African Americans to head a Fortune 500 company, according to the Executive Leadership Council.

Trump tweeted that at least Frazier will now ""

The response, and the speed in which it arrived, caught many off guard.

William Galston, a senior fellow at the Brookings Institution, said he couldn't "think of a parallel example" of any president responding as viciously as Trump to a CEO departing an advisory council.

"Usually, certain niceties are observed to smooth over a rupture," said Galston, who served as a domestic policy aide in the Clinton administration.

Within hours, Under Armour CEO Kevin Plank, who has felt some blowback for his support of the president, resigned from the same panel, Plank did not specifically mention Trump or Charlottesville, but said his company will focus on promoting "unity, diversity and inclusion" through sports.

But Intel CEO Brian Krzanich was more specific when he resigned a short time later,

The president followed up later in the day, tweeting that Merck ""

Drugmakers have come under withering criticism for soaring prices in the U.S., including by Trump, though he has yet to act on a promise to contain them.

The exchange lit up social media, with many people lauding Frazier and blasting the president. There was also a push online seeking more resignations from the remaining executives on the same panel, just over 20 of them.

Trump eventually made a statement condemning bigotry Monday afternoon at a press conference, but already, other executives came to Frazier's support.

Unilever CEO Paul Polman wrote on Twitter, "Thanks 螕åòMerck Ken Frazier for strong leadership to stand up for the moral values that made this country what it is."

Frazier was not the first executive to resign from advisory councils serving Trump.

Tesla CEO Elon Musk resigned from the manufacturing council in June, and two other advisory groups to the president, after the U.S. withdrawal from the Paris climate agreement. Walt Disney Co. Chairman and CEO Bob Iger resigned for the same reason from the President's Strategic and Policy Forum, which Trump established to advise him on how government policy impacts economic growth and job creation.

The manufacturing jobs council had 28 members initially, but it has shrunk since it was formed earlier this year as executives retire, are replaced, or, as with Frazier, Musk, Plank and Krzanich, resign.

"We've learned that as president, Mr. Trump is behaving exactly as he did as a candidate," Galston said. "He knows only one mode: When attacked, hit back harder."

AP Economics Writer Josh Boak in Washington contributed to this story