"The only war that is worse than one that's intended, is one that is not intended," Biden said. "This is not a game. This is not about you know can I puff my chest out."
Biden said he is disappointed that Trump has not become more presidential over the year, and that he believes the President is "much, much too cavalier" about his role in the world.
"It's not just a matter of manners, it's a matter of perception," Biden said. "Perception of whether the United States is being consistent. Whether the United States is still as stable and as steady a player in the world as it always been. I think it shows really poor judgment for the President to perform the way he does. Particularly with tweets. But not just with his tweets. Words matter."
He added, "The United States has a role in the world that the world has come to expect. I think the President is much, much too cavalier. And it's dangerous."
Biden, who returned to the Capitol for the swearing-in of Sen. Doug Jones of Alabama, said that the world sees in Trump a President acting in a way that is "totally inconsistent" with his predecessors.
"Let me choose my words, which is unusual for me," Biden said. "I think the President has to come to better understand that words matter coming from a president. There's a reason why it's important to be presidential. It's not just style. There's consequences. Severe consequences."
Biden did not raise this issue Wednesday when he saw Vice President Mike Pence on Capitol Hill because he says didn't think it was appropriate, other than to suggest to Pence that he "continue to give the President his advice."
On Tuesday evening, Trump tweeted
, "North Korean Leader Kim Jong Un just stated that the 'Nuclear Button is on his desk at all times.' 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!"
Biden said Trump's tweets have not played into his calculus on whether he will run for president in 2020.
"My decision to run for president, I have such profound disagreements from President Trump, if the only part of the calculus is whether or not I think he should still be president -- it wouldn't be close, in terms of my decision. But my decision is much more complicated than that. It's about whether or not for me in this moment in my life and in my families' life that it makes the best sense."
However, Biden was uncharacteristically quiet when asked if he trusted Trump with the nuclear codes -- smiling quietly and letting the silence speak for itself, then telling reporters, "I have to get on my old subway, guys."