Sen. Chris Coons (D-Del.) became the 33rd senator to announce support for the Iran nuclear deal Tuesday, just one shy of the 34 votes needed to uphold President Obama's veto of Republican legislation aimed at blocking the agreement. (AP)

Sens. Chris Coons (D-Del.) and Bob Casey (D-Pa.) have decided to support the Iran nuclear deal, putting President Obama on the brink of a major diplomatic breakthrough that will allow him to fully implement the controversial deal over the objections of the Republican-led Congress.

Coons's decision, disclosed in an exclusive interview with The Washington Post, delivered a powerful blow to opponents of the plan because the Delaware Democrat had previously voiced some of the deepest skepticism about the controversial deal. It came as Casey announced support for the deal and left Obama needing just one more vote in the Senate to claim victory.

Coons reached his decision after many weeks of deliberation that included long talks with top administration officials — including his political mentor, Vice President Biden — and an exchange of letters with President Obama that codified the assurances he received about the pact's implementation.

[New poll shows how sharply partisan the debate on Iran deal has become]

"We are better off trying diplomacy first," Coons told The Washington Post.

He will formally announce his position Tuesday afternoon at the University of Delaware, during which he is expected to explain the administration's position that this deal would give the United States a stronger long-term position over Iran because of the international backing it has. "They are saying what I need to hear," he said of Biden and other Obama officials.

Casey voiced similar sentiment. "This agreement will substantially constrain the Iranian nuclear program or its duration, and compared with all realistic alternatives, it is the best option available to us at this time," he wrote in an 8,000-word memo first shared with the Philadelphia Inquirer.

Coons and Casey become the 32nd and 33rd Democrats in the Senate to announce support, with just two Democrats opposed. Obama needs only one more Democrat to reach the 34 votes needed to uphold a veto of the Republican-drafted resolution to block portions of the accord.

When the Senate returns from its summer break after Labor Day, it will take up a resolution that would disapprove of the Iran deal — those voting yes, therefore, are opposed to the plan, while the nay votes are actual supporters.

Led by the American Israel Public Affairs Committee, opponents of the nuclear deal pooled tens of millions of dollars into a media campaign throughout the August congressional break designed to pressure lawmakers into opposing Obama's plan to lift economic sanctions in exchange for restrictions on Iran's nuclear program, notably its production of enriched uranium. The goal was to match all of the House and Senate Republicans with enough Democrats to reach a two-thirds majority to override Obama's veto of the resolution.

Instead, the anti-deal campaign has only been able to shore up the already expected opposition from Republicans. One by one, week by week, dozens of congressional Democrats have repeatedly declared their support for the deal over the past month, with just a few exceptions.

Here's what's in the Iran nuclear deal, and what happens next. (Gillian Brockell and Julio C. Negron/The Washington Post)

So much so that, with Coons joining their ranks Tuesday, Obama's allies in the Senate have shifted their game plan from winning the 34 Democrats needed to uphold the presidential veto, to also pushing for the support of 41 Senate Democrats so that the resolution would die in a filibuster.

There are now just 11 undecided Senate Democrats, and opponents need to get all 11 to oppose Obama in order to reach a veto-proof margin — an almost impossible task, given that several are core administration allies. Sen. Richard J. Durbin (Ill.), the No. 2 Democrat in leadership, has been the president's point person on the issue, and if Durbin wins over eight of the 11 remaining Democrats, he will have secured a double victory for Obama: filibustering the Republican-drafted resolution opposing the Iran deal and shielding the president from even using his veto pen.

In the interview, Coons said that this was one of the most difficult votes of his five-year tenure in the Senate, but not because of the AIPAC-led media campaign. Instead, Coons said, he has made calls and held face-to-face conversations with many friends and longtime political supporters who have strongly held beliefs against the deal.

He recently held a town hall at Wilmington's Jewish Community Center with more than 100 of northern Delaware's Jewish community leaders, an impassioned discussion on both sides of the issue. Afterward, Coons said, a friend of more than 20 years told him to never speak to him again if he voted for a deal the friend believed would put Israel in grave danger because of the potential of an Iranian nuclear weapon.

"This is, for them, a matter of life and death," he said, adding: "That's not light, that's not easy."

His main concern, Coons said this week, was the idea that Iran would be able to continue with its nuclear energy program for industrial power and wait out the accord's timeline, so that in 10 to 15 years, its leaders could switch to military capacity in a matter of weeks.

"That's the core concern," he said. "All they've got to do is be really patient."

Ultimately, Coons was convinced that this deal offered the best chance of keeping a strong international hand on Iran. In meetings in the Capitol basement, European diplomats assured him that they were ready to enforce the deal and that if in the coming years Iran cheated, the United States would have a united international front.

"We will be alongside you all the way," one diplomat said, according to Coons, while warning that that would not be the case if Congress rejects the deal and seeks military action in the near term: "You go it alone."

Casey voiced similar views that, if the deal is implemented well, it will leave the U.S. military in a stronger place to take action if the Tehran regime violates its tenets. "I believe this agreement must be undergirded by a clear and unequivocal statement from both the Administration and the Congress: we are prepared to take military action if Iran attempts to develop a nuclear weapon," said Casey, who like Coons, is a member of the Senate Foreign Relations Committee.

Despite a dozen hearings and many other private huddles, Coons still wasn't sure of his position about 10 days ago. "I got to a place where I wanted the advice and input of Joe," he said of the vice president, whose Senate seat Coons now holds.

Their long talk proved pivotal, and he decided to support the deal. During a mid-August congressional delegation trip to Africa, Coons paused while in Ethiopia to call Sen. Bob Corker (R-Tenn.), chairman of the Foreign Relations Committee, to debate the issue one final time with his friend.

Coons said the vitriolic depiction of the debate in TV ads and in social media was not evident in the many discussions he had with his voters.

"The people of Delaware are decent and thoughtful," he said.