President Donald Trump's actions toward North Korea, including taunting its leader, Kim Jong Un, with a tweet saying that his "nuclear button" was "bigger," was one of a series of actions that raises the "risk of unnecessary war," Sen. Tim Kaine said Wednesday.

"The president has had a steady pattern of undermining diplomacy," the Virginia Democrat, who was Hillary Clinton's runningmate in 2016, told MSNBC's "Andrea Mitchell Reports" program.

"If you do that, you raise the risk of unnecessary war, you make America less safe. So whether it's juvenile tweets, whether it's not even nominating an ambassador to South Korea, whether it's undercutting Secretary of State [Rex] Tillerson or Secretary of Defense [James] Mattis when they say the United States is never out of diplomatic options."

But instead, he continued, Trump engages in "provocative behavior that undercuts diplomacy and makes us unsafe."

In addition, Trump "can't just fire nuclear buttons," Kaine continued.

"He has to go to Congress," the senator told Mitchell. "[There are] no wars without Congress. That's what the Article I powers of Congress include, as probably our most solemn responsibility, and the fact that the president is sitting there tweeting, thinking he can just punch a button for nuclear weapons is not the way it works. That's why we are fighting back against him acting unlawfully."

A president can defend the United States against an imminent attack, Kaine continued, but the country has never used nuclear weapons in a first strike.

"That's sort of been an understood pillar of American foreign policy after World War II was over, from [Harry] Truman forward," Kaine said. "If he wants to do something preemptive, he's got to come to Congress first, and the president doesn't seem to understand that.

"He fired missiles at Syria in April without coming to Congress and still hasn't given us any legal rationale for that missile strike. The thought that he could think his nuclear button is big and he has the power to do something without Congress should trouble everybody."

Meanwhile, the president has spoken in support of protesters against Iran's government, and Kaine said lawmakers should be involved in discussions with the White House in favor of sanctions. Further, he said he's worried about Trump's call for the United States to step away from the Iran nuclear deal, which by all accounts is working.

"If the president steps back from a deal that's working, why would North Korea entertain getting into a diplomatic deal with the United States to limit its nuclear weapons program?" said Kaine. "They have to believe if we do a deal, we will stick to a deal, and I think the biggest danger of what the president is doing in Iran right now is he's sending a signal to North Korea that North Korea could not trust the United States to live up to a nuclear deal."