I don’t know who wrote the letter, but I can tell you whose fingerprints are on it: the only folks who are supporting it publicly, the hard-right Israel lobby. Even as Cotton himself splutters on national television, rightwing lobby groups are the main voices out there defending the letter.
Like Bill Kristol of the Emergency Committee for Israel:
(snip)
There’s a reason for Williams’s suspicion. Kristol’s Emergency Committee for Israel gave Tom Cotton nearly $1 million in his race for the Senate just five months ago, Eli Clifton reported. “Cotton received $960,250 in supportive campaign advertising in the last month.” (Thanks to Kay24 in comments).
Cotton also got $165,000 from Elliott Management Paul Singer’s hedge fund. Singer is the billionaire who is trying to stop Obama’s Iran talks (Clifton’s reporting again). He funds the Israel Project too– Josh Block’s efforts.
Follow the money. This group also believes that this agreement if reached with Iran is only temporary and will not out last Obama's term.