Cotton is joined by Republican Sens. Joni Ernst (Iowa), Mike Rounds (S.D.), James Lankford (Okla.) and Thom Tillis (N.C.), Rabbitt said. The senators will return to the United States Friday evening.
Cotton has defended using the Guantanamo facility to hold prisoners. He attracted attention when he remarked at a recent Armed Service Committee that "the only problem with Guantanamo Bay is there are too many empty beds" and that terrorists can "rot in Hell, but as long as they don't do that they can rot in Guantanamo Bay."
Cotton touched off a firestorm with his Monday letter, which was directed at Iranian leaders with the intention of derailing a potential nuclear deal with the country. Cotton, who opposes the talks and says he does not trust the Iranians, warned that any long-term deal would need the approval of Congress, not just President Obama.
Obama and his fellow Democrats have sharply criticized the letter. He has also taken heat from some Republicans.
Speaking at a breakfast with Bloomberg reporters and editors, Sen. Ron Johnson (R-Wis.) said, "I suppose the only regret is who it's addressed to,'' adding, "But the content of the letter, the fact that it was an open letter, none whatsoever."
Forty-six Republican senators signed the letter while seven did not. No Democrats signed it.
The liberal group MoveOn.org Civic Action launched a TV ad Friday hitting the senators who signed on. "this level of sabotage is unprecedented," says the narrator.