Asked by Baier if the president should be impeached, Steyer said: "I think he's the most corrupt president in American history. I think he's more than met the criteria for impeachment. I think he threatens the safety of Americans -- but more than that, he threatens the whole idea of the rule of law."
Steyer, a billionaire businessman and founder of San Francisco investment firm Farallon Capital, added he believed no president should operate "above the law."
If elected president, Steyer said, he'll declare a national "state of emergency" on his first day in office to deal with what he called the threat from global warming.
"On Day 1, I would use the emergency powers of the presidency to start us to deal with the issue of climate change, to make it Priority 1 and to make sure that we protect the health and safety of every American."
However, he called the Paris climate agreement -- from which the U.S. withdrew under Trump -- an "important, symbolic gesture, but not actually something that's going to change what the world does or what America does."
"The Paris accord is by no means perfect," he said.
"If we're going to solve it, we're going to have to solve it on a global basis."
He maintained the U.S. should be at the forefront of combating the climate-change issue.
Additionally, when it comes to his presidential campaign platform, Steyer told Baier his bid is centered around a somewhat populist message.
"We've had a corporate takeover of our federal government," he said.
"They've actually bought the government and that we need to restore our democracy and to get back to government 'of, by and for the people' that puts the interest of the American people first, not the profits of corporations.