(NPN) – South Dakota Republican Senator John Thune joined 82 of his U.S. Senate colleagues in a letter to President Barack Obama Tuesday outlining what they believe should define any final agreement with Iran on its nuclear weapons program.

All Northern Plains states’ Senators joined in the letter except Democrats Tim Johnson of South Dakota and Tom Harkin of Iowa.

The letter was headed by Senators Mark Kirk (R-Ill.), Robert Menendez (D-N.J.), Lindsey Graham (R-S.C.), Charles Schumer (D-N.Y.), Chris Coons (D-Del.) and Kelly Ayotte (R-N.H.).

In their letter, the Senators write: “We must signal unequivocally to Iran that rejecting negotiations and continuing its nuclear weapon program will lead to much more dramatic sanctions, including further limitations on Iran’s exports of crude oil and petroleum products.”

The letter also noted the Senators’ “core principles” concerning any deal with Iran.

North Dakota Senators Heidi Heitkamp (Dem.) and John Hoeven (Rep.), Nebraska Senators Mike Johanns and Deb Fischer (both Rep.), Wyoming Senators Michael B. Enzi and John Barrasso (both Rep.), Minnesota Senators Al Franken and Amy Klobuchar (both Dem.), Montana Senator Jon Tester (Dem.) and Iowa Senator Chuck Grassley (Rep.) also joined the letter.