DES MOINES — Iowa’s Republican U.S. senators defended sending a letter to Iranian officials telling them any agreement with President Barack Obama could be reversed by another president “with the stroke of a pen.”

“I consider it a letter of education about how our constitutional system works,” Sen. Chuck Grassley told reporters Wednesday. “I think it’s a good thing to educate the Iranian people about the difference between agreement and a treaty under the Constitution.”

He described a treaty, which must be approved by two-thirds of the Senate to be ratified, as having the same effect as law.

“No one questions the fact that a president can make an agreement, but that agreement can changed the same way the president made an agreement,” Grassley said.

Ernst, who also joined 46 other Republican senators in signing the letter, said Congress has the authority “to weigh in on any Iran nuclear agreement.”

“And we should weigh in,” she said in her conference call with reporters.

It’s in the America’s best interest that a long-standing agreement between with Iran is approved by Congress, Ernst said. “We know they continue to develop enriched uranium and I do not believe it is for peaceful purposes.”

She called a nuclear Iran “a grave threat to our nation and allies in the region” including Israel.

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Ernst rejected arguments that the senators’ letter weakens America’s position in negotiations or is undermining the president.

“It clearly spells out our intentions,” she said.

Grassley, who noted he studied diplomatic history while a student at Iowa State Teachers College — now the University of Northern Iowa, he called for a common-sense approach to the issue.

“What harm is it going to do?” he asked. “I haven’t seen any harm yet.

“If it’s going to cause some harm, let’s deal with it at the time of the harm.”