Senate Minority Leader Harry M. Reid (D-Nev.). (Carlos Barria/Reuters)

Senate Minority Leader Harry M. Reid gave a forceful endorsement Sunday to the nuclear deal with Iran, a key boost that provides continued momentum for preventing Congress from blocking President Obama’s pact.

The Nevada Democrat said the deal, which lifts economic sanctions against the rogue nation for pledges to limit its nuclear program, is the “best way” to curtail Iran’s military ambitions, and he pledged to round up more support to thwart its opponents.

“I’m going to do everything in my power to make sure the deal stands,” he said in a telephone interview with The Washington Post from his home in Henderson, Nev., where he has been calling friends to tell them of his decision.

The retiring Democratic leader becomes the 27th Senate Democrat to publicly endorse the plan, while just two — including Reid’s friend and likely successor, Sen. Charles E. Schumer (N.Y.) — have come out against it.

Where lawmakers stand on the Iran deal

That whip count, along with many private conversations he has had with undeclared Democrats, led Reid to pronounce he was “cautiously optimistic” that he would be able to secure enough support to prevent an override of Obama’s veto of a resolution opposing the deal. An override would require opponents mustering 13 Democrats to join all 54 Republicans in opposition to the president.

Reid said he was “still hopeful” that at least 41 Democrats would support Obama, which would suffocate the resolution in a filibuster and not require a presidential veto.

Reid acknowledged that he has been supportive of the deal for many weeks now, holding regular conversations with White House Chief of Staff Denis McDonough on the details of the deal and the congressional review process.

“This is the best way, the only way, to keep Iran from getting a nuclear weapon,” Reid said Sunday.

He rejected the assertions from Schumer and other longtime supporters of Israel that the deal does not guarantee global investigators enough access to sites where Iran has worked on nuclear development.

“We don’t disagree on much, but we disagree on this,” he said of Schumer.

He cast Schumer’s opposition as a “vote of conscience” that would not harm his standing in the Democratic caucus, even though a vast majority support the deal.

Reid said he owed it to many of his supporters to come back to Nevada and talk to them over the congressional break, including some who opposed the plan. Sheldon Adelson, the Las Vegas casino magnate and major Republican donor, is financing some of the opposition campaigns against the deal.

Reid declined to discuss his conversation with Adelson, with whom he has a friendly relationship. “I’ll keep that to myself,” he said.

Adelson and supporters of the American Israel Public Affairs Committee, the most prominent opponent of the deal, had hoped to use the five-week break to increase pressure on wavering Democrats through advertising campaigns as they visited voters back home. So far, that effort has fallen flat, particularly in the Senate, where many Democrats had initially expressed reservations about the nuclear pact.

Since the Senate adjourned nearly three weeks ago, more than 15 Democrats have announced their support, including a handful in conservative-leaning states who seemed to have been potential targets to oppose Obama.

Under the deal, Obama will be able to lift sanctions that he and previous presidents imposed on Iran. But the remaining sanctions, imposed by Congress, will have to wait until the review process is finished. By Sept. 17, the House and Senate will hold votes on resolutions disapproving of the Iran pact, and there is clear majority support in each chamber to reject Obama’s deal because of near-universal Republican opposition.

Opponents of the deal with Iran must ensure that they reach a two-thirds majority in each chamber to overcome an Obama veto of the resolution of disapproval.

Reid’s optimism in sustaining the deal is grounded in the numbers: 17 Democrats have not declared their intentions on how they will vote, and Reid needs to persuade just seven to support Obama to ensure that at least 34 senators would sustain the veto, keeping it below a two-thirds majority.

The leader just hosted two of the undecided Democrats, Sens. Michael F. Bennet (Colo.) and Patty Murray (Wash.), both up for reelection next year, at a fundraising weekend in Nevada’s Lake Tahoe region. He declined to discuss private talks with his colleagues, while pledging to coax Democrats to support the president on the Iran vote.

“I’m going to do everything I can,” Reid said.