The 'arguments' being made by most conservatives subscribing to this thread are ridiculous, idiotic, and reprehensible.
The right's attempt to 'justify' the letter because they perceive the government in Iran as 'bad' or 'untrustworthy' is just as reckless and irresponsible as the moronic letter sent by 47 republican dullards.
Instead of having the courage to admit that the letter was wrong and a mistake most on the partisan right seek to act as apologists for those who sent the reckless and irresponsible letter only because they're fellow republicans and because they share an unwarranted hostility toward the president.
Rightly or wrongly, the letter-signatories cannot be touched, nor will they be.
The President has lost the trust of much of Congress and the trust of much of The People, to conduct the foreign policy of the United States.
Especially in matters relating to striking a potential bad and appeasing deal with a self-declared mortal enemy of the United States, obtaining nuclear weaponry to affix atop its existing medium-range ballistic missile delivery systems.
People still remember Neville Chamberlain, and the lessons learned from that well-intentioned yet naive and gullible fellow's appeasing behaviors and outcomes.
People have long-since come to associate the President with such behaviors, especially with respect to Muslim adversaries - fairly or unfairly, rightly or wrongly.
The Senate at-large, as a co-equal branch of the US Government, and as the body responsible for vetting and ratifying treaties, is not barred from influencing the negotiation of treaties, and is seen by a great many as undertaking an 'intervention' in this instance, to either (1) thwart attempts to reduce this to an Agreement that does not have to be vetted nor ratified by the Senate, and to (2) thwart attempts to serve-up a Chamberlain-esque 'bad deal' about so visceral an issue as nuclear weaponry in the hands of our mortal enemies.
Chances are very good that each and every one of those Senators who signed the letter did so with at least a modicum of regret that we had come to such a sorry state of affairs, but, they may have acted from the courage of their convictions, so, rightly or wrongly, they get props for that much.
The rest will be proven or disproven by future events, and, in the meantime, there will be no indictments nor sanctions leveled at those signatories.
And, they have served notice to our Lame Duck President that he is perceived as no longer trustworthy, to conduct the foreign affairs of The Nation, unsupervised.