This administration works in secrecy and have been found at times to be less than truthful. Thus there is a great deal of mistrust. This is compounded by keeping the aspects of the negotiations secret except for leaks.
This has led to the frustration being shown by the writing of the letter. They don't trust Obama, via Kerry, to make a good deal. What has leaked out seems to have confirmed those fears. Here we have a commander-in-chief who is solely responsible for foreign policy who has yet to keep his word on any of the conflicts hammering the Ukraine and much of the middle east. Why, they ask, should they trust him on this?
As Iran shows off it's new missiles and takes shots at a mock up U.S. carrier during war games, why would we think their nuclear ambitions should be given any leeway? They are openly hostile to the United States. They are basically annexing Iraq by leading their troops in the war against ISIS and if the suspected ethnic cleansing is really happening around Tikrit that opens a new front of the war.
Sunni ISIS against Shite Iranians and Iraqis. As the Shites plow through Sunni territory they must refrain from killing those left behind as ISIS retreats or the conflict will evolve into a full fledged civil war. One that has been smoldering long before we were even a country is now on the brink of exploding.
A nuclear Iran could make short order of their Sunni problem, ISIS would no longer be competing for the 'califate' and they'd be free to go after the rest of we infidels in the most deadly of ways.
This is what some in Congress wonder if the President grasps. If his ambition is to have an agreement that allows a time frame for Iran to go nuclear with no further restrictions, in time they will carry out their own ambitions. Does the President care? Does the supposed glory of an agreement mean more than a relatively safe world?
The Senators who signed the letter know well by now how the President's mind works. They rightly worry about it. They are letting the world know that the President does not speak for all of us. No advice was sought, no 'go ahead' given by anyone who knows the territory, the history of Iran, the dangers being invited and the mind set of the President.
They are worried. Israel and Egypt and Jordan are worried. We too should be.