Sen. Chris Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Del.) says President-elect Donald Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s tweets could spark global chaos.

“We can’t have a president who tweets one thing at 5 a.m. and then his press people walk it back at 7 and then he tweets something else at 9 a.m. and then his secretary of State walks it back at 10 a.m.,” he said on CNN’s “New Day” Friday.

"That’s just going to lead to chaos in our international relations.”

“I’m very concerned with how the president-elect is conducting himself,” added Coons, who endorsed Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE. "If I have one piece of advice for him, it’s put the phone down and stop tweeting about vital national security issues.”

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Trump on Thursday tweeted about expanding the U.S. nuclear arsenal but didn't share what inspired the tweet.

“The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes,” he said.

His tweet came the same day that Russian President Vladimir Putin, also called for his nation to “strengthen [its] strategic nuclear forces."

The arms-control community greeted both remarks with alarm Thursday, raising fears of a new nuclear arms race.

Sean Spicer, Trump’s incoming White House press secretary, on Friday downplayed such concerns, however.

“[T]here’s not going to be [an arms race] because he is going to ensure that other countries get the message he is not going to sit back and allow that,” he reportedly said on NBC’s “Today.”