The number of executives exiting the White House's American Manufacturing Council this week rose to four on Tuesday as the CEOs from Intel and Under Armour and the President of the Alliance for American Manufacturing tendered their resignations. Their decisions come one day after Merck CEO Kenneth Frazier made his exit. 

Frazier cited the president's failure to explicitly rebuke the white nationalists who marched in Charlottesville, Virginia, over the weekend. He wrote on Twitter Monday that "America's leaders must honour our fundamental values by clearly rejecting expressions of hatred, bigotry and group supremacy, which runs counter to the American ideal that all people are created equal."

Frazier is one of the few African-Americans to head a Fortune 500 company.

Under Armour CEO Kevin Plank resigned from the panel later Monday, saying his company "engages in innovation and sports, not politics." Plank did not specifically mention Trump or Charlottesville, but said his company will focus on promoting "unity, diversity and inclusion" through sports.

Intel CEO Brian Krzanich followed, writing in a blog post that while he had urged leaders to condemn "white supremacists and their ilk," many in Washington "seem more concerned with attacking anyone who disagrees with them."

"It is clear even to me that nearly every issue is now politicized to the point where significant progress is impossible," Krzanich added. "Promoting American manufacturing should not be a political issue.”

 Alliance for American Manufacturing President Scott Paul also tendered his resignation from the Council, stating it's: 'the right thing for me to do'.

Trump said Tuesday afternoon that CEOs who have quit his White House council are "leaving out of embarrassment."

He also said they are "not taking their jobs seriously" as he takes questions from reporters at Trump Tower in New York City, where he is spending a few days during a break from Washington.

Trump was under increasing pressure to call out the white supremacist groups involved in the Charlottesville demonstration. He lashed out almost immediately at Frazier, saying on Twitter that he will now "have more time to LOWER RIPOFF DRUG PRICES!" 

On Tuesday, Trump took to Twitter again to announce he has "many" CEOs that could take the place of the ones who left. 

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.

With the barbs, Trump appeared to attack an industry executive who has tried to make drug pricing somewhat more transparent by revealing his company's overall drug price changes.

In January, Merck reported that its average net prices -- the amount the company receives after discounts and other rebates -- increased in the years since 2010 in a range between 3.4 per cent and 6.2 per cent per year. That's about half as large as the increase in its retail prices. Much of the furor over drug prices recently has been over increases that have been far bigger and come one after another for drugs that have been on the market for years.

The exchange lit up social media early Monday, with many people lauding Frazier and blasting the president. Trump eventually made a statement condemning bigotry Monday afternoon at a press conference.

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Meanwhile, other executives stated their support for Frazier.

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

Frazier, who grew up in a poor neighbourhood in Philadelphia, resigned from the manufacturing advisory council days after one person was killed and others wounded in violent clashes between white supremacists and protesters.

Frazier and his siblings were raised by their janitor father after their mother died when they were very young. He has earned a reputation as a risk taker in the drug industry, pouring money into daunting research areas, particularly trying to develop a treatment for Alzheimer's disease.

Frazier is 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 and Musk, resign.

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.

"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."