A letter to the Iranian government seeking to undermine President Obama's nuclear negotiations with Iran drew 47 Republican signatures and widespread derision from Democrats and the White House.

"Beneath the dignity of an institution I revere" is how Vice President Biden described the GOP letter. Here are the seven Republicans who didn't sign on and some of their reasons for abstaining.

Sen. Bob Corker (R-Tenn.), Senate Foreign Relations Committee chairman: "Sen. Corker's focus is on getting a veto-proof majority to support his bipartisan bill for congressional review of any comprehensive nuclear agreement with Iran," said an aide to the senator.

Sen. Jeff Flake (R-Ariz.): Flake's spokeswoman, Bronwyn Lance Chester, told CNN that the senator agreed with the spirit of the letter but abstained from signing it because he did not "believe the letter was necessary."

Sen. Susan Collins (R-Maine): “I don’t think that the ayatollah is going to be particularly convinced by a letter from members of the Senate, even one signed by a number of my distinguished and high-ranking colleagues,” the senator told reporters.

The other Republican senators who didn't sign are: Lamar Alexander (R-Tenn.), Lisa Murkowski (R-Alaska), Dan Coats (R-Ind.) and Thad Cochran (R-Miss.).