Seventy-five former lawmakers are urging Congress to back the Iran deal ahead of next month's vote, calling it the "most viable means" to block Tehran from getting a nuclear weapon.
"We believe that the Joint Comprehensive Plan of Action that the United States and our P5+1 partners negotiated with the government of Iran is the most viable means to prevent Iran from acquiring a nuclear weapon and protect the security of the United States, Israel and other allies," they wrote in the letter, sent on Monday. "We are writing to you as former Members of Congress to urge your support of this historic agreement."
They added that there is "no viable alternative" to the agreement and that Congress should back the deal and then use their "oversight role throughout the robust monitoring and evaluation of the implementation."
Levin's successor in the Senate, Sen. Gary Peters (D-Mich.), is still publicly undecided on the Iran nuclear deal.
Separately, Lugar added that "sticking with our international partners in a unified and systematic effort to ensure Iran's compliance with this agreement is the best opportunity we have to block an Iranian nuclear weapon, while avoiding a costly war and preventing additional countries in the region from seeking a nuclear arsenal."
Monday's letter is the latest example of former lawmakers weighing in on the political fight, as both opponents and supporters try to sway their former colleagues on the Iran nuclear deal.
Congress has until Sept. 17 to vote on the deal and until early October to override a potential veto from President Obama.
Obama will need 34 senators to back the deal and uphold a potential veto for the agreement to survive Congress.
Though Republicans have lined up against the deal, the president is close to securing a victory, with 31 senators backing the agreement, according to The Hill's whip list.
At least two additional Senate Democrats, Sens. Bob CaseyBob CaseyTen senators ask FCC to delay box plan Lawmakers blast poultry, meat industries over worker injuries GOP chairman sees funding deal soon on medical cures bill MORE (Pa.) and Chris CoonsChris CoonsDems: Warren ready to get off sidelines Dems pressure Obama on vow to resettle 10,000 Syrian refugees Wildlife crime bill deserves unanimous consent in Congress MORE (Del.), are expected to announce their decisions this week.
The former lawmakers added in the letter that, as former members of Congress, they "appreciate the gravity of your decision."
"Supporting the agreement does not come without risks. But these risks must be considered in the context of the grave risks that would be incurred if you were to reject it," they said. "These include the unraveling of international sanctions, the suspension of international inspections and an Iranian government that is unconstrained in developing its nuclear program. They also include the increased likelihood of a military confrontation."