Republican Sen. Tom Cotton sparred Wednesday with Hillary Clinton on Twitter, after the former secretary of state slammed an open letter he and other GOP senators sent to Iran's leaders about ongoing nuclear talks.

The Republican senators' statement has become the subject of immense controversy, as it challenged President Obama's authority to strike a nuclear deal and cautioned Iran's leaders that any agreement would need congressional approval in order to necessarily last beyond Obama's term.

Clinton, who is widely expected to enter the presidential race in the coming months, tweeted a warning to those potential Republican candidates who have praised the letter.

Arkansas Sen. Cotton fired back.

Louisiana Gov. Bobby Jindal also entered the fray, tweeting:

Democrats have slammed the Republicans' letter as a diplomatic no-no, claiming they effectively undermined the U.S. president on the world stage. Vice President Biden said earlier this week that "the decision to undercut our President and circumvent our constitutional system offends me as a matter of principle."

Secretary of State John Kerry echoed those concerns during congressional testimony on Wednesday, while also disputing the senators' assertion that U.S. lawmakers could simply alter any nuclear agreement years later -- because, he said, it is not technically a "legally binding plan."

Rather, he described the pending deal as an executive agreement, which needs no congressional approval.

Republicans, though, have stood by their decision to fire off the letter -- part of a campaign to demand a vote in Congress on any nuclear deal. A bipartisan bill is pending in Congress that would do just that. The senators claimed in their original letter that while the administration is pushing an "executive agreement," the agreement would have more heft if it were approved by Congress.

Clinton, meanwhile, has weighed in on the letter controversy at the same time she's dealing with her own controversy about her use of personal email while secretary of state. She opened her press conference on Tuesday about the email issue by, first, condemning Republicans over the Iran letter.