...Sending a letter to the nation that the administration is negotiating with is asinine as it breaks down the negotiations...
Sending a letter to an unaccountable and martyrdom-encouraging theocratic junta which labels us The Great Satan, that the American People see through their bullshit, even if our present-day Neville Chamberlain ghost doesn't (or doesn't care) - and putting them on notice that whatever Neville negotiates, it won't be allowed to stand beyond January 20, 2017, probably IS intended to break down the negotiations.
As the maxim goes, a bad deal is usually far worse than no deal, so - best to throw a monkey-wrench into the works before the Mistake gets too much traction.
...Tell Obama that the agreement will not be ratified and fought until certain conditions are met and leave it to the president to actually negotiate the deal...
My guess is, that this possibility was considered first, and dispassionately, and for a long time, before reaching the conclusion that our petulant President would have gone ahead and done it his way, regardless, so... why leave the negotiations to someone who is determined to head down the wrong path, regardless of what others tell him?
...This move looks bad on the republicans...
Or, alternatively, it gives the Pubs the aura of rescuers or champions of the situation... stopping an Appeasement President from giving away the farm; a way of saying to Liberals: "OK, kiddies, play-time is over. You've had six years, and you still haven't figured-out this Iran thing. You're too dangerous to be allowed to continue unsupervised. We'll take it from here, until your shaky lease on the Oval Office has expired. Thank you."
...it highlights a fundamental misunderstanding of international politics...
Or, alternatively, it highlights a fundamental understanding of international politics - or, at least, a fundamental understanding of such as it applies to this particular foreign nation (Iran)?
...Do you think that, after this letter was sent, Obama is now in a STRONGER position to make demands against Iran or WEAKER?...
Weaker, of course.
But that's (quite probably) the point of the exercise, isn't it?
The Congress (and a sizable percentage of The People) no longer trust the President in such matters, and perceive him to be negotiating unfavorable terms, and would much rather torpedo current negotiations and revert to the status quo until more competent leadership can be brought to bear on the subject after January 20, 2017?
It is clear that the President has lost the confidence of much of Congress and much of the American People.
Once that happens, other strange things follow, such as hobbling the Lame Duck as a matter of damage control and prevention, until he leaves office.
This is an example of such hobbling of a Lame Duck, apparently... rightly or wrongly... for better or worse.
...Answer that, truthfully, and you can see why this is a blatant error.
That depends upon whether one trusts the President and perceives him to be competent and acting in the best interests of the United States in such matters, I expect.
You appear to do just that.
A great many others no longer do, if, indeed, they ever did.