“We need a commander in chief who knows how to protect America and our allies and advance our interests and values around the world. The stakes are high,” the group, including former top U.S. nuclear negotiator Wendy Sherman, wrote in a letter released Tuesday. “We are concerned that Senator Sanders has not thought through these crucial national security issues that can have profound consequences for our security.”

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President Obama opposes normalized relations, as does Clinton, although Obama had campaigned against Clinton in 2008 on a platform that included an offer of dialogue with Iran.

As Obama’s first-term secretary of state, Clinton agreed with the strategy to initiate secret diplomacy that ultimately led to the nuclear deal. She supports the broad international deal reached last year to rein in Iran’s nuclear program in exchange for billions in international sanctions relief. Clinton has not hidden her deep skepticism about Iran and her generally hawkish approach, however, and has raised public doubt about Iran's commitment to upholding the agreement.

During Sunday’s NBC-YouTube debate, Sanders was asked whether he would support the reopening of a U.S. embassy in Tehran and the establishment of normal diplomatic relations, which ruptured after Iranians stormed the American Embassy in 1979 and took diplomats and other personnel hostage.

“I think what we have got to do is move as aggressively as we can to normalize relations with Iran, understanding that Iran’s behavior in so many ways is something that we disagree with,” Sanders replied.

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He cited Iran’s alleged support for terrorism and “the anti-American rhetoric that we’re hearing from some of their leadership.” He added that the nuclear agreement and the better U.S.-Iranian relations forged during its negotiation were accomplished “without going to war.”

Now it’s time to go further, he said.

“Can I tell you that we should open an embassy in Tehran tomorrow? No, I don’t think we should. But I think the goal has got to be as we have done with Cuba to move in warm relations with a very powerful and important country in this world.”

Obama, who as a candidate had also pledged to try to bridge the decades long rift with Cuba, has overseen a swift normalizing of relations there over the past 13 months. The U.S. flag was raised over the embassy in Havana last August.

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The letter from Clinton supporters, some of whom worked in the Bill Clinton administration, does not mention Cuba. But it questions whether Sanders understands the complex international dynamics surrounding Iran, including the role of U.S. allies.

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“We are all strong supporters of the nuclear diplomacy with Iran. Some of us were part of developing the policy that produced the diplomacy over the past several years. And we believe that there are areas for further cooperation under the right circumstances,” the group wrote.

But Sanders “is out of step with the sober and responsible diplomatic approach that has been working for the United States,” by supporting a much warmer relationship, it said.

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And his call for a closer partnership between Iran and Saudi Arabia “is just puzzling,” given the two nations' intense rivalry and current estrangement, the group wrote.

The strategy “would fail while causing consternation among our allies and partners.”

Iran has also given no sign that it seeks or would accept a much warmer relationship with the United States, although pragmatist Iranian President Hassan Rouhani has seemed to welcome Obama’s limited outreach.

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The group also took issue with Sanders for his strategy against the Islamic State in Syria and Iraq.

“His lack of a strategy for defeating ISIS -- one of the greatest challenges we face today -- is troubling,” the group wrote. “And the limited things he has said on ISIS are also troubling.”

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Clinton is seeking a departure from current White House strategy against the Islamic State, including a no-fly zone and other extensions of U.S. might currently opposed by Obama, but the letter does not examine those differences.

“Given these concerns, it is important to ask what he would do on other issues -- on Russia, China, our allies, nuclear proliferation, and so much else,” the group said of Sanders.

“We need a commander in chief who sees how all of these dynamics fit together — someone who sees the whole chessboard, as Hillary Clinton does.”

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A Sanders spokesman issued a response late Tuesday.

“Senator Sanders has a lot of respect for the senior U.S. diplomats who support Secretary Clinton and question his views on ISIS and Iran. We certainly concede that former secretary of state Clinton has more experience than Senator Sanders, but his judgment on major foreign policy issues is far superior," spokesman Michael Briggs said.

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He noted that Sanders voted against the Iraq war in 2002 while Clinton voted for it, and he said he agrees with Obama that a no-fly zone in Syria opens the door to dangerous consequences.

"As president, Senator Sanders would do all that he could to destroy the barbaric Islamic State terrorist group, but he will do it by maintaining a strong coalition of major powers and our Muslim allies. He agrees with King Abdullah of Jordan that it must be Muslim troops on the ground that destroy ISIS with the support of the United States and other major powers providing air support, training and military equipment," Briggs said. "He will do everything he can to prevent U.S. troops from getting involved in perpetual war in the quagmire of the Middle East."

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