Sen. Chris Coons (D-Del.) said on Tuesday that he will support the Iran deal, bringing President Obama to within one vote of securing a major diplomatic victor. (Getty Images)

President Obama is within one vote of securing the biggest diplomatic victory of his presidency, now that Sens. Chris Coons (D-Del.) and Bob Casey (D-Pa.) have committed themselves to support the deal to curb Iran's nuclear ambitions.

Coons's decision came in an exclusive interview with The Washington Post, in which he said that the country is "better off trying diplomacy first," while Casey said the Iran agreement is "the best option available to us at this time," in a sprawling memo first shared with the Philadelphia Inquirer.

From the Post's Paul Kane:

Coons's decision… 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…

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.

[Chris Coons and Bob Casey back Iran deal, putting Obama one vote from major diplomatic victory]

Coons and Casey are the 32nd and 33rd Senate Democrats to support the deal. Obama needs a total of 34 Democrats to sustain his veto of a resolution of disapproval that the House and Senate are expected to vote on later this month.

By The Washington Post's count, Obama should have no problem reaching that threshold, as some still-undeclared senators have spoken positively about the deal. There are still eight senators who remain undecided and have not tipped their hand as to how they would vote.

[President Obama is one senator away from clinching the Iran deal]

The Republican-led House is expected to easily pass the disapproval resolution, but if 41 senators support the Iran deal, a growing possibility, then they could band together and filibuster the resolution, preventing the president from having to issue a veto.

Karoun Demirjian covers defense and foreign policy and was previously a correspondent based in the Post's bureau in Moscow, Russia. Before that, she reported for the Las Vegas Sun as its Washington Correspondent, the Associated Press in Jerusalem, the Chicago Tribune, Congressional Quarterly, and worked at NPR.