"He's made repeated threats. He's tested missiles time and time again for years," White House press secretary Sarah Huckabee Sanders said of Kim. "This is a president who is not going cower down and is not going to be weak and is going to make sure he does what he's promised to do, and that's stand up and protect the American people."
Trump's tweet Tuesday was highly unusual for its open reference to the vast U.S. nuclear arsenal at Trump's disposal and its playground allusion — critics said puerile — to the size and power of Trump's warfighting abilities.
"North Korean Leader Kim Jong Un just stated that the 'Nuclear Button is on his desk at all times,'" Trump tweeted. "Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works!"
Trump was responding to a New Year's message from Kim that included an offer to talk with his enemy South Korea — a U.S. ally — and a threat that he has a "nuclear button" on his desk and could threaten the United States.
Democrats were quick to condemn Trump's tweet and called on him to stop ratcheting up tensions with the rogue nation.
"A nuclear conflict on the Korean Peninsula would be a catastrophe, leading to the deaths of potentially millions of people, including American servicemembers and families stationed there," Sen. Edward J. Markey (D-Mass.) said in a statement, adding that the tweet "borders on presidential malpractice."
"We cannot let this war of words result in an actual war," Markey said.
Senate Minority Leader Charles E. Schumer (D-N.Y.) said Wednesday that Trump's recent Twitter outbursts presented "a very poor representation of the United States" and may represent "a new low point."
The North Korea threat came on top of Trump tweets over the past few days levying threats of cutting aid to Palestinians and Pakistan, as well as tweets taking credit for civilian aviation safety, attacking the press and suggesting an aide to his Democratic former rival Hillary Clinton should be jailed.
"President Trump's foreign-policy-by-tweet is doing serious damage to the country," Schumer said in an address on the Senate floor. "Where we have serious issues to address abroad, President Trump seems happy with macho boasts and belligerent threats that get us nowhere."
Schumer suggested that congressional Republicans are complicit if they do not speak up, but most Republican lawmakers remained silent or downplayed the seriousness of Trump's tweet.
"I just took it as him basically pushing back on Kim Jong Un and saying: 'You've been trying to have this bravado because you have a weapon that may be able to reach us. Understand we have a lot more force,' " said Sen. James Lankford (R-Okla.) "We should actually have some sort of negotiations on this rather than just have bravado on this."
Vice President Pence, interviewed Wednesday on Voice of America, touted Trump's record in encouraging greater cooperation from China, North Korea's most important ally and economic lifeline, to apply economic pressure on the isolated regime.
"President Trump has provided the kind of clear leadership on the world stage that's made measurable progress," Pence argued.
"President Trump made it clear," Pence added. "America will not be bullied. America will not be threatened."
Reaction was muted overseas, with close U.S. allies South Korea and Japan making no public mention of the tweet.
A senior State Department official said Secretary of State Rex Tillerson had a routine diplomatic phone conversation with his South Korean counterpart late Tuesday.
"Diplomacy remains our preferred approach. That has not changed," the official said. "But we recognize that diplomacy has to be backed up by credible force."
South Korea and North Korea held their own call hours after Trump's message — the first such use of a cross-border hotline in two years.
The State Department official, who requested anonymity to discuss sensitive diplomacy, welcomed the contact, perhaps as a means to ease tension ahead of South Korea's hosting of the Winter Olympics in a few weeks.
The North Korean offer of some kind of talks with Seoul has been widely interpreted as a play to drive a wedge between the United States and the new, less hawkish South Korean government.
"We are in close contact" with South Korea, the official said. "We are on the same page and coordinating closely," but "extremely skeptical" that North Korea is serious bout embarking on talks aimed at giving up its nuclear weapons, the official said.
Some foreign policy analysts and Trump critics said Trump's tweet raised questions about whether he is creating a foreign policy crisis — and about his fitness for the job.
"If there are any adults at the WH: please disable the @realDonaldTrump smartphone immediately," tweeted Daryl G. Kimball, executive director of the nonprofit Arms Control Association. "Nuclear diplomacy shouldn't be conducted as if its a junior high school argument about penis size. Such reckless tweeting diminishes U.S. credibility and can lead to catastrophe."
Sanders rejected such criticisms.
"I think the people of this country should be concerned about the mental fitness of the leader of North Korea," she said at the White House daily briefing.
She also argued that the Trump administration is being forced to "clean up" former president Barack Obama's international messes.
"This is not a game. This is not about, can I puff my chest out bigger than your chest,'" former vice president Joe Biden told CNN when asked about Trump's tweet. "This is just not," Biden continued, his voice trailing off. "It's not presidential."
Karoun Demirjian contributed to this report.