Hillary Clinton slammed Republican lawmakers over their letter to Iran warning Congress could undo a nuclear deal. Clinton also warned 2016 hopefuls not to sign onto it. "GOP letter to Iranian clerics undermines American leadership. No one considering running for commander-in-chief should be signing on," she tweeted Wednesday, a day after she took reporters' questions about her exclusive use of a private email server while she headed the State Department. Presumptive GOP 2016 candidates Ted Cruz, Marco Rubio and Rand Paul, along with 44 other GOP senators, have all signed the open letter -- in which they suggest they can undo any nuclear deal the Obama administration might agree to.