Coons’s decision, coming 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.

“We are better off trying diplomacy first,” Coons told The Washington Post in an exclusive interview.