The letter by Senator Tom Cotton, and signed by 46 other Republican senators ("Cotton letter") is the second time that Republicans have violated the existing protocols on how the U.S. conducts foreign policy. As is often the case, much of the argument I have seen revolves around whether such action is treason or a violation of law, the contention being that if it is not prohibited by law then it is perfectly permissible.
This ignores the fact that something as complex as government requires respect for unwritten conventions and respect for the institution as a whole to function. Republicans appear to have made a concerted opinion to undermine the Executive Branch on the exercise of foreign policy well beyond the usual, unwritten bounds. The Cotton letter is extraordinary in that it is a public statement addressed to a foreign power by a substantial number of Senators to undermine the negotiating ability of the President. As with Boehner's invitation to Netanyahu to address a joint session of Congress, it is technically permissible (nothing stops Senators, in the exercise of their duties, from saying publicly how they plan to vote in the event a treaty is placed before them for ratification), but destructive of the institution and undermines our ability as a country to conduct foreign policy.
The best case for Republicans consistently violating established protocols on foreign policy -- is that they believe that once a deal is reached it will be impossible to exercise the standard checks and balances. Because such a deal will be honored by the other members of the 5+1 negotiating team, and because the Obama Administration will permit the U.N. to lift sanctions in accordance with the agreement, the traditional means by which Congress shows its disapproval (rejecting the treaty) will not prevent Iran from achieving nuclear capability. Therefore, from the standpoint of Republicans, in light of the extraordinary danger (in their view) to the U.S. and its allies in the Middle East (which includes not merely Israel, but most other Sunni regimes that are unhappy about the prospect of Iran having a nuke), they have no choice but to use every means at their disposal to prevent an agreement from occurring at all.
This is not inherently irrational. But it does have costs, as noted above. It is, to use what has become a common term in politics but ironically appropriate here, a "nuclear option" for the conduct of foreign policy that will extend well beyond the issue of Iran, or the current Administration.
Unfortunately, my observation over the last 15 years has been that Republicans seem to operate on the principle that if Democrats (or progressives generally) *object* to such tactics when Republicans use them, that Democrats are somehow honor bound to refrain from engaging in them when the tables turn and the political winds shift. This misconceives the reason we have rules, traditions and protocols to govern these relationships. Rules are only worthwhile if they apply equally to everyone. A system where some participants feel bound by rules and others do not is a dysfunctional system that advantages the rule breakers. The entire reason for having such rules is because both parties consider them necessary for the ability of government to function. They are not a moral imperative, but an artificial construct to facilitate a common goal -- even when operation of a rule may prove inconvenient in the short term.
Anyone who can remember back to their days in elementary school can recall what happens when someone decides to change the rules of a game in the middle. The other players respond. Because what is "fair" is when the same rules apply equally to everyone. In the grownup world, this is why actions in violation of unwritten rules and conventions (or even in violation of written rules) have potentially broader consequences.
One of the key points of the Federalist Papers was the need for a strong executive to carry on a unified foreign policy. At the same time, Federalist Papers also recognized the role of Congress in preventing the Executive from acting with complete disregard of the concerns of the states and the representatives of the people. The complicated set of unwritten rules on the manner in which Congress exercises its oversight of the Executive in foreign policy, even where they appear to people to be utterly arbitrary and irrelevant (for example, the difference between a letter signed by 46 Republican Senators and public statements by individual Senators amounting to the same thing), are nevetheless critically important in maintaining the smooth operation of government -- especially divided government.
That said, we should keep in mind that the system of government in the United States is strong, that we have on occasion seen the balance of power shift between the Congress and the Executive Branch on foreign policy or domestic policy, and that such shifts are arguably sometimes justified and necessary. The two that come to my mind are the post-Vietnam era -- when Congress sought to curb the power of the executive to conduct military operations without a declaration of war and rein in perceived abuses by intelligence agencies, and the period after World War I, when Republicans rejected the League of Nations, defunded much of the military and retreated into isolationism. In each case, a strong argument can be made that the Executive branch had exceeded its authority and that to prevent abuse it was necessary to recalibrate the balance of power on foreign policy so that Congress could retain proper oversight as intended by the Constitution.
Regardless of the substance of the policy, however, and whether one agreed or not, both periods were marked by a substantial decline in the United States' ability to influence world affairs as a consequence of the internal tug-of-war. It may be that Republicans sincerely believe that the Obama Administration's conduct of foreign policy on Iran is so drastically wrong, but if so they need to fully understand the consequences for the next decade. The alternative, that they are deliberately undermining the conventions of foreign policy for short-term political gain, undermines the ability of the U.S. to manage coherent foreign policy and has wide-ranging repercussions.