"The President is about to impose on himself and this country a dramatic self-inflicted wound because by pulling out of this agreement, Iran will go back onto a path to develop a nuclear weapon," the Connecticut Democrat said on CNN's "State of the Union."
If the US exits the agreement, Iran would still receive sanctions relief from other nations party to the deal, and the move would also make Tehran "look like the victim," Murphy said.
"They will get everything they want," he said.
In a televised interview with former Arkansas Gov. Mike Huckabee that aired Saturday night, Trump also wouldn't say if the US was planning to get rid of the Iran deal.
"In a few days from now, a week and a half to be exact, you'll see exactly," Trump told Huckabee. "But I can tell you I'm very unhappy with the deal. I'm very unhappy with their attitude. ... Iran is a bad player. And they're going to be taken care of as a bad player."
Two senior US officials told CNN
last week that Trump plans to "decertify" the agreement and leave its fate up to Congress.
Murphy also criticized Trump for undermining the nation's diplomatic efforts, especially those of Secretary of State Rex Tillerson.
"We have two different foreign policies," Murphy said, one conducted by Tillerson and other top administration figures, the other -- a competing one -- from Trump's Twitter feed.
Trump tweeted last week that Tillerson was "wasting his time"
trying to negotiate with North Korean leader Kim Jong Un over that country's nuclear weapons program, writing, "Save your energy Rex, we'll do what has to be done!"
The tweets came a day after Tillerson said the US had direct lines of communication with North Korea and that he was trying to "calm things down" following months of escalating rhetoric over Pyongyang's nuclear weapons and ballistic missile tests.
Speaking at a press conference in Beijing, Tillerson said the US made it clear through its direct channels to North Korea that it was seeking peace through talks.
"We've made it clear that we hope to resolve this through talks," Tillerson said.
"I think the most immediate action that we need is to calm things down," Tillerson added. "They're a little overheated right now, and I think we need to calm them down first."
Asked about Trump's own rhetoric, Tillerson said the entire situation was "overheated."
Murphy said Sunday in his CNN interview that while he did not think Tillerson was a good secretary of state, anyone would have a difficult time with Trump's behavior.
"I have a feeling that whoever replaces Tillerson would suffer the same problem," Murphy said.
Tillerson delivered a statement
on Wednesday affirming his support for Trump and desire to stay in the role, following an NBC report that he called
Trump a "moron."
The report came just days after Tillerson said the US had direct lines of communication with North Korea to deescalate the nuclear crisis only to have Trump dismiss the efforts
Trump went further
on Saturday, tweeting, "Sorry, but only one thing will work!"
Asked about the tweets, Senate homeland security committee Chairman Ron Johnson said on "State of the Union" that Trump was right, in a sense.
"What's been tried in the past hasn't worked," Johnson said.
The Wisconsin Republican advocated for the US to get China "fully engaged" and eventually achieve "regime change," but without military force.
"There is no viable military option," Johnson said.