47 Senate Republicans
just sent a letter to the supreme leader of Iran, reminding him that they don't like him, and have some power to reject a nuclear deal because -> constitution.
Why would Republicans try to sabotage a deal with Iran?
Well, many reasons.
If I had to guess, I'd say the fundamental dynamic underlining this letter is that it's a part of a GOP strategic calculation along these lines:
-If Obama can get a good deal, when push comes to shove no one will block it.
-The Senate may not pass it, but anything reasonably strong will probably stay alive, simply because at the end of the day (some) republicans are perfectly aware invading Iran is a bad idea.
-Republicans think they have more to lose should they appear weak in front of their base than if they attack the deal and it still is passed.
-If the deal fails, it can be blamed entirely on Obama and most of the public will be none-the-wiser
It's not always easy to discern the difference between ideological blinders and genuine democratic-political necessity: a politician who makes idiotic decisions could be doing so because he/she is an idiot, or because that decision is genuinely what their constituents want and will help them politically. Obviously it's always some of both, but it's difficult to untangle the two.
I'm going a bit off the reservation here, but if I had to guess, I'd say that many establishment republicans are less concerned with the possibility of Iran becoming a nuclear state then they would ever admit. For one, Israel has been crying wolf for so long that "being concerned with Iran" is more of a political shibboleth than an actual worry. When it comes to the hard political realities, I don't buy it that all 47 of these senators actually want to sabotage the nuclear deal. Some probably think this nonsense won't actually matter when it comes down to it, but god forgive them if pass up the chance to score short term political points in the mean time.
Ideology definitely plays into this though. In the mind of the right, the only reason the US wouldn't be able to stop Iran from getting a nuke is weakness on Obama's part. You might call this "Reagan Syndrome", where all it takes to defeat our enemies is to stand strong, call out the axis of evil for the evil things they are (along with lots of patriot chest-thumping), and they will fold because they cannot stand up to our democratic perfection and military prowess. All you need to do is shout "Mr. Gorbachev, tear down this wall!" and it will single-handedly bring down the USSR.
That's my armchair-psychoanalysis, at least. Mind you, sometimes threats and force are warranted and useful, but republicans tend to go off the reservation when the country is led by a democratic president. Their bias is that democrats are weak, always, forever, regardless of the circumstances. 'Obama is playing into Iran's hand, how could he be so naive? He must be selling out Israel because he hates them' etc. etc.