Murphy, a member of the Foreign Relations Committee, had previously suggested that he was leaning toward supporting the agreement.
Leahy, meanwhile, suggested that without the deal Iran would be more likely to develop a nuclear weapon. He said rejecting the ageement would isolate the United States from the countries who participated in the diplomatic push.
"They stuck with us because we thought we were negotiating in good faith and we'd have a deal. If we walk out now, many of these countries are going to say, 'okay, you're in it by yourself,' " Leahy said.
"This is not a bad deal," he added.
The two endorsements come the day after Sens. Tim KaineTimothy (Tim) Michael KaineDemocrats call on Biden administration to ease entry to US for at-risk Afghans Manchin, Sinema join GOP to sink filibuster change for voting bill Desperate Dems signal support for cutting Biden bill down in size MORE (D-Va.), Barbara BoxerBarbara Levy BoxerBass raises nearly million since launching LA mayor campaign Harry Reid, political pugilist and longtime Senate majority leader, dies Congress can prevent another Jan. 6 by updating a key elections law MORE (D-Calif.) and Bill NelsonClarence (Bill) William NelsonOvernight Energy & Environment — Earth records its hottest years ever Global temperatures in past seven years hottest ever observed, new data show NASA welcomes chief scientist, senior climate adviser in new dual role MORE (D-Fla.) also threw their support behind the deal.
Senate Republicans have largely lined up against the Iran pact, which means Obama will need the support of at least 34 Senate Democrats to uphold a potential veto of legislation disapproving of the agreement. Many Senate Democrats, including Minority Leader Harry ReidHarry Mason ReidThe Hill's 12:30 Report - Presented by Connected Commerce Council - Biden faces reporters as his agenda teeters Biden hits one-year mark in dire straits 'All or nothing' won't bolster American democracy: Reform the filibuster and Electoral Count Act MORE (D-Nev.), are still undecided.
Opponents of the deal are hoping to use the five-week break ahead of a vote to pressure roughly a dozen Senate Democrats to buck the president, but with more Democrats backing the deal before leaving town, it could be an uphill battle.
Republicans, including the party's presidential contenders, have taken issue with multiple parts of the agreement, including "side" deals between Iran and the IAEA, the lifting of an arms embargo and that it doesn't require Iran to recognize Israel or release Americans from custody.
Murphy sought to combat that criticism on Wednesday, saying that while the Iran nuclear agreement wouldn't impact other key U.S. criticisms of Iran, "America and our allies will still fight the Iranian regime tooth and nail on their support for terrorists, their constant threats to the U.S. and Israel, and their denial of political and human rights to their own people."
He also dismissed another key Republican criticism, that Congress could walk away from the agreement and press for a better deal, as "pure fantasy."
"This deal has many unsavory elements," he said. "[But] I understand that the nature of a negotiation by definition involves not getting everything you want. ... The agreement has flaws, but the prospect that a better deal would result from congressional rejection seems like pure fantasy to me."
— This story was updated at 12 p.m.