Greta Van Susteren challenged Sen. Tom Cotton (R-Ark.) on why he, along with 46 other Republican senators, chose to write a letter to Iran's leaders. 

In Cotton's highly controversial letter, which was slammed by Hillary Clinton and the Obama administration, the Republicans cautioned Tehran that a deal on its nuclear program could be revoked by the next U.S. president.

Cotton then fired back at Clinton...

Cotton explained Tuesday on The Kelly File that the senators felt they needed to explain the U.S. Constitution to Iran's leaders, including the fact that any deal that does not go through Congress will not hold up. 

Greta took issue, however, with the strategy, arguing that Republicans could have published an open letter to Iran in the New York Times or some other high-profile newspaper. 

"Why do you have to become pen pals essentially with Iranian leadership? ...  Here, it does look like you're end-running the president. The president is end-running the Senate, he says he's not gonna get your approval. But why are you doing that?" she asked.

Cotton defended the "open letter to the leaders of the Islamic Republic of Iran" as showing exactly where Republicans stand on the issue in a "crystal clear" way.

Greta pushed back that Cotton could have achieved the same thing with an open letter in a newspaper, which would have still been read by Iran's leaders and brought less criticism.

Cotton answered that he has no regrets about what they did.

"We’re trying to stop Iran from getting a nuclear weapon and Iran has to understand that the Congress will protect America from a bad deal, as they have been doing for 200 years and as our founding fathers envisioned," he said 

Watch the full exchange above. 

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