In case if anyone is wondering which Republicans DIDN'T sign this:
Lamar Alexander
Dan Coats
Thad Cochran
Susan Collins
Bob Corker
Jeff Flake
Lisa Murkowski
Not surprised about Murkowski, Collins, or Flake. Corker seems to be more focused on trying to get his bill through, and Alexander seems to be piggy-backing off of that. As for Cochran and Coats, I was pleasantly surprised.
It could always be that some just forgot to sign or weren't able to sign yet. For example, 44 of the 46 Democrats signed a marriage equality letter. While Manchin clearly opposes gay marriage, I have no doubt that Tom Udall supports it.