NICHOLAS KAMM via Getty Images Hillary Clinton has touted her role in paving the road for the U.S. to engage in diplomacy with Iran, but revelations about her support for sanctions in the middle of nuclear negotiations raise questions about her diplomatic chops.

WASHINGTON -- A key talking point of Hillary Clinton’s presidential campaign is the story of how she laid the groundwork for the nuclear agreement with Iran during her time as Secretary of State by convincing the international community to join the U.S. in hitting Iran with crippling sanctions. While her role in sanctioning Iran is well-documented, it is less clear whether her ability to apply pressure on Iran, a long-time U.S. adversary, would have translated into an ability to bring about the diplomatic accord finalized last year.

Clinton was “skeptical” of negotiating with the Iranians from the outset, the New York Times reported on Monday. In the lead up to the 2008 election, she accused her rival, then Sen. Barack Obama (Ill), of naiveté for his offer to meet with U.S. adversaries without preconditions. She later agreed to meet with Omani intermediaries in 2011, but remained more cynical than her boss that negotiations would produce an agreement favorable to the U.S.

Clinton left the State Department in 2013, and was succeeded by Secretary of State John Kerry, who oversaw a series of diplomatic breakthroughs that culminated in the July 2015 nuclear deal. Clinton's exact role in the broader diplomatic effort with Iran has become a pivotal question as she moves closer to clinching the Democratic nomination, because it offers an indication of her ability to preserve the fragile nuclear agreement as president.

Clinton supports the nuclear agreement and her Iran policy is similar to the White House's, though hers is packaged with more hawkish rhetoric. But, according to The Times, she split with Obama and Kerry shortly after she left the administration on whether to ramp up sanctions against Iran in the midst of nuclear negotiations.

In December 2013, Sens. Mark Kirk (R-Ill.) and Bob Menendez (D-N.J.), introduced a new sanctions bill against Iran that attracted bipartisan support. The Obama administration urged lawmakers to hold off on the bill, arguing that the timing was terrible.

Hassan Rouhani, a far more moderate politician than his predecessor, had been elected president in Iran earlier that year, campaigning on a pledge to restore the country’s economy by negotiating with the West to lift sanctions. Iran, the U.S., and its five negotiating partners had secured an interim agreement on Iran’s nuclear program the previous month. New sanctions, the Obama administration argued, were unnecessary and would demonstrate to the Iranians that the U.S. was not negotiating in good faith.

J. Scott Applewhite/ASSOCIATED PRESS Secretary of State John Kerry warded off past efforts by Congressional Republicans and Democrats like Sen. Bob Menendez (N.J.) to pass new sanctions against Iran while negotiations with Iran over its nuclear program were ongoing.

Publicly, Clinton backed the White House at the time, writing to lawmakers that they should “give diplomacy a chance to succeed.” But privately, she was influenced by several lawmakers and Israeli Prime Minister Benjamin Netanyahu, who predicted that more sanctions would force the Iranians to cave on their demands, The Times reported.

“She would have squeezed them again,” an unnamed source who worked with Clinton for several years told The Times.

Ultimately, Obama and Kerry convinced lawmakers to hold off on that round of sanctions, a feat that may have been complicated if there was discord between the president and the Secretary of State.

It’s impossible to say what effect Clinton would have had on the nuclear negotiations had she remained in office. Her allies suggest that her willingness to ramp up the pressure against Iran wouldn’t have precluded her from reaching an agreement with the long-time U.S. adversary -- and actually might have pressured the Iranians to offer more concessions. Her detractors say that her approach would have presented political challenges for Rouhani to continue the talks and could have tanked the negotiations entirely.

Proponents of U.S.-Iran diplomacy have voiced concern in the past over the fate of relations between the two countries if she succeeds Obama. “I am worried about her instinct,” Trita Parsi the head of the National Iranian American Council, told The Huffington Post in January. “She is far too inclined to think that only pressure works.”

Since the nuclear agreement came into effect earlier this year, politicians in Iran and the U.S. have already alleged noncompliance by the other side.

Critics have accused Iran of violating the deal by conducting ballistic missile tests (although the United Nations resolution in question only calls on Iran to refrain from testing ballistic missiles designed to carry nuclear weapons). Iranian Foreign Minister Mohammad Javad Zarif has said that the U.S. hasn’t done enough to reassure the international community it won’t be punished for doing business with Iran, minimizing the benefits of sanctions relief.

If Clinton succeeds Obama as president, she will inherit the task of making sure the nuclear deal doesn’t crumble under all this pressure. She has already proven her ability to aggressively monitor Iran’s nuclear activity and punish the country for non-compliance. But this will be the first test of her willingness to ward off efforts by hawkish lawmakers -- some of whom were her allies in the Senate -- to pass new sanctions that could threaten the agreement.