Former secretary of state Hillary Rodham Clinton, having embraced the Iran nuclear deal negotiated by President Obama, is now helping to lead the sales effort for it — both with her own party and the electorate at large.

The 2016 Democratic presidential front-runner’s endorsement stands against nearly unanimous Republican opposition, led by denunciations from the large and growing field of GOP candidates. The clash offers further evidence that foreign policy could loom as a crucial issue in the election.

Clinton’s endorsement also underscores the degree to which her political fortunes have been joined with Obama’s legacy. That is an ironic turn, given how the two tangled over Iran during the 2008 race for the Democratic nomination, with Clinton dismissing Obama’s talk of overtures to Iran as “irresponsible and, frankly, naive.”

But after serving as Obama’s chief diplomat during his first term, Clinton took credit Tuesday for “having been part of building the coalition that brought us to the point of this agreement.” In a series of private meetings and a lunch with Democrats on Capitol Hill, Clinton spoke supportively of the deal.

“Based on what I know now, and I will be being briefed as soon as I finish addressing you, this is an important step in putting the lid on Iran’s nuclear program,” Clinton told reporters.

Where she had once been skeptical of whether the nuclear negotiations had any chance of success, Clinton said the key questions now are the degree to which the agreed-upon terms are enforced and whether Iran continues its “bad behavior” in other areas, including sponsoring terrorism, undermining other governments in the Middle East, human rights violations and threatening Israel.

The Republican candidates, on the other hand, offered near-apocalyptic assessments of the agreement, which will go forward unless Congress can muster two-thirds veto-proof majorities to oppose it in both houses.

Sen. Ted Cruz (Tex.) called the deal a “fundamental betrayal of the security of the United States and of our closest allies.” Sen. Lindsey O. Graham (S.C.) told MSNBC that it ensures “that the Arabs will go nuclear” and is “a death-over-time sentence to Israel if they don’t push back.” Former Florida governor Jeb Bush said it “paves Iran’s path to a bomb.”

But where opposition to a deal has been a surefire applause line as the GOP candidates have stumped before conservative audiences in early primary states, influential Republican leaders in Congress took a somewhat more measured tone.

“I want to read the agreement in detail and fully understand it, but I begin from a place of deep skepticism that the deal actually meets the goal of preventing Iran from obtaining a nuclear weapon,” Senate Foreign Relations Committee Chairman Bob Corker (Tenn.) said in a statement.

Sen. Lindsey Graham (R-S.C.), a long-shot GOP presidential candidate, blasted the Obama administration’s nuclear proliferation deal with Iran. (Alex Wong/Getty Images)

Notably silent, at least at first, was Sen. Rand Paul (Ky.), a presidential contender from the non-interventionist libertarian wing of the GOP.

“The small-government group doesn’t want him to be a knee-jerk militarist. I don’t think he wants to be, either,” said Drew Ivers, who was Iowa chairman for the 2012 presidential campaign of Paul’s father, former congressman Ron Paul (R-Tex.). “But it’s a can of worms for him, whether to show thoughtful statesmanship versus dried-out partisanship.”

Reservations on Capitol Hill were not confined to Republicans.

“I’m concerned that the deal ultimately legitimizes Iran as a threshold-nuclear state,” said Sen. Robert Menendez (N.J.), a Democrat on the Foreign Relations Committee. “I’m concerned the red lines we drew have turned into green lights; that Iran will be required only to limit rather than eliminate its nuclear program.”

If Clinton were elected, she would have to implement the deal, which is one reason her assessment carries such weight with her fellow Democrats.

She is “one of the two most important and most influential voices of this debate, the other being President Obama,” Rep. Steve Israel (D-N.Y.), a longtime Clinton supporter, said after the former secretary of state’s Tuesday morning presentation to the House Democratic caucus. Israel said he remains “skeptical” of the Iran deal.

Public polls show that majorities or pluralities of Americans support the broad outlines of a deal with Iran that would lift economic sanctions in exchange for restrictions on or inspections of its nuclear program. But surveys also indicate that most do not trust Iran to abide by the terms of an agreement.

Duke University professor Peter D. Feaver, who served on the National Security Council staff in the administrations of Bill Clinton and George W. Bush, said that as debate over the deal goes forward, the political advantage is likely to rest on whichever side frames the choice.

Obama will prevail if Americans consider the Iran deal an alternative to further military conflict in the region, Feaver said. But Republicans could win the argument if they are convinced that tougher negotiations could have produced a better deal, he added.

“It’s a jump ball, and it’s not untethered to facts on the ground,” Feaver said, adding that Iran’s behavior going forward also will be a factor.

Not to be upstaged by Clinton, Sen. Bernie Sanders (I-Vt.), a fellow presidential candidate, strolled out of a Democratic lunch with her and told reporters he welcomed Clinton “back to the United States Senate.” He went on to sketch out the areas where he and Clinton disagree, including on trade policy, climate policy, financial regulation and her past support for the Iraq war.

Sanders — who has been drawing large crowds of progressives on the campaign trail — said he said hello to Clinton in the luncheon but did not question her. “I’ve known the secretary for 25 years,” he said. “I like her, I respect her, and I hope that we can run a campaign where we can express the differences of opinion that we have and do it in a way that is straightforward.”

Robert Costa, Jose A. DelReal, Mike DeBonis and Peyton M. Craighill contributed to this report.