White House spokesman Josh Earnest told reporters that a letter the senators sent to Iran was an attempt to "throw sand in the gears" of sensitive negotiations as the U.S. and five other world powers try to reach a basic agreement with Tehran by March 31.

　　"I would describe this letter as the continuation of a partisan strategy to undermine the president's ability to conduct foreign policy and advance our national security's interests around the globe," Earnest said.

　　White House Accuses Republican Senators of Undermining Iran Talks

　　Freshman Republican Senator Tom Cotton of Arkansas organized the letter, which was signed by the party’s entire Senate leadership - but no Democrats.

　　The letter said the leaders of Iran “may not fully understand” the U.S. Constitutional system, and that anything not approved by Congress is a mere executive agreement. The senators wrote that “the next president could revoke such an executive agreement with the stroke of a pen.”

　　Obama reacted quickly to the letter, saying he thinks it is somewhat ironic to see some members of Congress wanting to make common cause with hardliners in Iran - calling it an unusual coalition.  The president said what his administration is focused on right now is seeing whether it can get a deal as part of the international “P5+1” talks with Tehran in Switzerland.

　　Obama said if a deal is reached, he will be able to make a case to the American people and is confident he will be able to implement it.

　　Iranian Foreign Minister Javad Zarif also was quick to respond to the letter, saying it has no legal value and is mostly a propaganda ploy.  Zarif expressed astonishment that some members of the U.S. Congress find it appropriate to write to the leaders of another country against their own president and his administration.

　　Responding in a tone similar to the tone used in the senators’ letter, Zarif said “the authors may not fully understand that in international law, governments represent the entirety of their respective states, are responsible for the conduct of foreign affairs, and are required to fulfill the obligations they undertake with other states.”

　　Several Democratic lawmakers said they were appalled by the Republicans’ letter. Democratic Senate Minority Leader Harry Reid said the letter was sent to embarrass the president, and he accused Republicans of trying to undermine their commander-in-chief as he seeks to conduct foreign policy.

　　Republican lawmakers say Congress needs to have a say in a final deal over Iran’s nuclear program - with a deadline for a political framework set for the end of March.

　　Republican Senator Bob Corker and Democratic Senator Robert Menendez have a bill pending that would mandate congressional review of an Iran nuclear deal, but Democrats have insisted that they want to wait and let the international talks play out before the bill is brought to the floor for a vote.

　　Republican House Speaker John Boehner sparked a firestorm of controversy when he invited Israeli Prime Minister Benjamin Netanyahu to address a joint session of Congress last week on the dangers of an Iran nuclear deal - without consulting the White House.

　　Democrats were divided over the speech, with House Minority Leader Nancy Pelosi saying it was condescending, and that she was near tears during the speech.

　　After the speech was over, Obama reminded Congress that foreign policy is conducted by the executive branch, and not the legislative branch.

　　Over the weekend, Obama said the U.S. will "walk away" from the nuclear talks with Tehran if it decides that an internationally verifiable deal cannot be reached to keep Iran from building a nuclear weapon. But he has threatened to veto proposed legislation that would mandate congressional review of any deal that is reached.

　　Tehran insists that its nuclear program is for civilian purposes and has been negotiating with the U.S., Germany, Britain, France, China and Russia on the scope of nuclear activities it can carry out in exchange for lifting the crippling economic sanctions the U.S. and Europe have imposed on it.

　　Negotiators are facing a self-imposed deadline to complete the basic structure of a deal by March 31, with final agreement by the end of June.