Donald Trump's hardline proposal was attacked as impractical and divisive by John Kasich and Jeb Bush.
The 10 candidates in the evening's main debate were anxious to stand out, and several were quick to attack.
Democratic presidential candidate Hillary Clinton endured a testy 11-hour hearing in Congress.
"While I will not be a candidate, I will not be silent," stated U.S. Vice President Joe Biden.
Forty Democrats and two independents voted to block a resolution disapproving of the pact.
Republicans, however, pledged to keep up their fight against the pact with new sanctions on Tehran.
Billionaire celebrity candidate backpedaling after caustic remarks about female debate moderator.
Sen. Chuck Schumer opposes President Obama, saying Iran would use deal to pursue "nefarious goals."
President Barack Obama is pushing to seal the Iran nuclear deal, accusing opponents of "selling a fantasy."
The French president and his American counterpart will meet in Washington on Tuesday to discuss strategy.
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