Though the letter is unequivocal about the GOP's opposition to the bill, it does not contain an explicit threat to use the Senate rules to block debate on the bill, if Reid tries to bring it to the floor.
But though the letter does not draw that particular bright line, it markedly stiffens the caucus' position. Minority Leader Mitch McConnell made waves Tuesday by blasting the Dems' proposal, claiming it would institutionalize taxpayer funded bailouts of Wall Street. The assertion was widely disputed by both Democrats and experts, and more than 24 hours later, several Republicans were unwilling to endorse McConnell's take. One of them, Sen. Bob Corker (R-TN), even suggested GOP leadership should tone down its rhetoric and try to reach consensus with Democrats.
Now, it seems, they've all decided to endorse that line.
You can read the full letter here.