U.S. Sen. Ron Johnson, R-Wis., says he signed an open letter warning Iranian leaders against reaching a nuclear agreement with the U.S. because he thinks President Barack Obama has mishandled negotiations.

The letter, drafted by Sen. Tom Cotton and signed by a total of 47 Republican senators, encourages Iran to back away from ongoing talks that could result in restrictions on that country's nuclear program. The letter has come under fire from Democrats and some political commentators who see it as inappropriate from a constitutional perspective as well as an attempt to sabotage Obama's diplomatic efforts.

On Tuesday, Johnson sent Wisconsin Public Radio a statement that said he decided to sign the letter with the nation's best interests at heart.

"I agreed to sign Senator Cotton’s open letter because I believe it describes reality — it tells the truth," Johnson wrote in the statement. "It also argued that any long-standing agreement in the best interest of the American people should be ratified by their representatives in Congress."

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He wrote that he feels Congress should "have a say in whether or not this deal is good for America. That judgment should not be left to President Obama acting alone."

The senator wrote that the United Nation's Security Council has passed resolutions during the last decade calling on Iran to suspend its enrichment, but that Obama had "moved the goal posts by suspending sanctions that brought Iran to the negotiating table." He added that the president is mistaken for "acknowledging Iran’s right to enrich uranium. As a result, the negotiation was lost before it even began."

Johnson echoed sentiments expressed by Israeli Prime Minister Benjamin Netanyahu during his speech before Congress last week that the proposed agreement "paves the way for Iran to get a nuclear weapon."