More or less, the occupant of the White House personifies U.S. foreign policy. Describing a president as strong or weak is often our shorthand way of evaluating American activity abroad.

But in fact, presidents are the wrong yardstick. We should speak of a strong Washington or, as is the current state of affairs, an enfeebled one.

Which brings us to that letter to Iranian officials, offered up this week by 47 Senate Republicans who suggested that it was futile for Ayatollah Ali Khamenei, that country’s supreme leader, to cut a nuclear deal with President Barack Obama.

“The next president could revoke such an executive agreement with the stroke of a pen and future Congresses could modify the terms of the agreement at any time,” the letter intoned.

It was simply the latest international advertisement of a Washington so divided that it’s willing to sacrifice the nation’s clout on serious, life-and-death matters. So says Sam Nunn, the former U.S. senator from Georgia and one of the world’s foremost experts on nuclear proliferation.

“I think the United States is being weakened around the world because of the perception and reality of an ideological split and profound distrust between the president and the Congress,” Nunn said in a Wednesday interview.

We had not talked since his daughter, Michelle Nunn, lost her Democratic Senate bid to Republican David Perdue in November.

The letter to Iran was authored by Tom Cotton, the recently elected Republican senator from Arkansas. But Perdue was the second signer. Johnny Isakson added his name, too.

Sam Nunn stayed away from any personal attacks. In fact, he never mentioned the names of Georgia’s two Republican senators. Instead, he focused on the dangers posed by a Congress attempting to pre-empt a president in international affairs.

“Whatever the intent of the letter, and I don’t try to divine that intent, it’s being interpreted around the world as a large part of the Republican Senate suggesting that the Ayatollah should not trust or make agreements with a U.S. president,” Nunn said.

“To say the least, it’s a very significant departure from the norm of American foreign policy.”

Nunn said he has detailed content concerns about any agreement to keep Iran from obtaining nuclear weapons. He does not quarrel with arguments that Congress has an important role, and sympathizes with senators who say they feel cut out of the administration’s efforts to keep Iran from obtaining nuclear weapons.

“That criticism has been made often enough, including by some Democrats, that I think they make a legitimate point,” Nunn said. “But I also believe Congress has to try to be constructive, and not destructive. A president isn’t going to trust a Congress that announces its intent to basically stop everything he’s doing.”

Specifically, Nunn worries about the impact the Iranian letter might have on the disparate coalition – which includes Russia, France, China, Britain, and Germany – behind the economic sanctions that have driven Iran to the negotiating table.

“That embargo has been enormously successful. For 20 years, we’ve gone with Iran continuing to make progress toward making a nuclear weapon. The embargo is the first thing that’s really gotten their attention. And I think the Obama administration deserves some credit for that,” Nunn said.

But should the nuclear talks fail, whether because of opposition in Washington or Tehran, the coalition would have to agree to keep economic sanctions in place. Nunn didn’t say as much, but the implication is that some members of the coalition could point to the Iranian letter from Congress as evidence of bad faith and walk away.

“That plays right into the Iranian hands. That’s their goal. If something breaks down – they’re goal is to split this very effective economic coalition,” Nunn said. “I’m afraid what we’ve done is play right into Iranian strategy.”

Without an embargo, military force becomes the only means of keeping Iran from going nuclear. Otherwise, four or five other countries in the Middle East would quickly follow suit.

I asked Nunn about House Speaker John Boehner’s decision to invite Israeli Prime Minister Benjamin Netanyahu to address a joint meeting of Congress.

Nunn says he wouldn't have boycotted the speech. But on the whole, he disapproved – for the same reason he thinks the Iranian letter a bad idea. "I did not think it was productive either for the U.S. or Israel," Nunn said. "It demonstrated the split. We need to demonstrate cohesion between key friends. Again, it weakens our leverage vis a vis Iran."

Toward the end of our conversation, Nunn offered up a tidbit that I hadn’t heard before. On several trips to Russia, he would meet with the country’s minister of atomic energy and complain of Russia’s cooperation with the Iranian nuclear program.

“I learned not to do that, because he would get out charts and show me in great detail that the Iranian nuclear program started under the Shah with Western technology and Western cooperation,” Nunn said. “We forget a little bit of that history.”