WASHINGTON—President Barack Obama locked in enough support in Congress Wednesday to ensure he can overcome bipartisan opposition and implement a landmark nuclear accord with Iran.

More than a month after global diplomats struck an agreement limiting Iran's nuclear activities in exchange for international sanctions relief, the White House secured the backing of 34 Senate Democrats—the minimum needed to guarantee the deal can advance despite deep and divisive reservations in Congress, especially among Republicans.

Even if Congress passes a resolution disapproving the deal when it votes later this month, Mr. Obama is expected to veto the resolution. Support from Sen. Barbara Mikulski (D., Md.), who is retiring next year, means there will not be enough votes to override that veto. The president is also expected to secure the votes needed in the House, though he needs to sustain his veto in just one chamber to proceed with the deal.

The moment marked a victory for an administration that has lobbied fiercely to protect the foreign policy capstone of the president's second term from equally determined opposition. The level of Senate support effectively blunts the effort of Republicans and some Jewish groups, led by the American Israel Public Affairs Committee, to derail the accord.

But Mr. Obama's apparent victory also comes at a cost. He is now poised to implement a critical piece of American foreign policy without support from the majority of Congress. The Iran deal only has partisan support from a minority of Democrats in Congress, while a bipartisan majority opposes it.

The endorsement gave Mr. Obama a cushion of support broad enough to win a looming veto fight with Congress. Under legislation enacted in May, both chambers are required to vote by Sept. 17 if they want to pass a resolution approving or disapproving the agreement. Republicans, who control both chambers, have prepared disapproval legislation, which is expected to pass the House and face a tight vote in the Senate, where Democrats hope to block it on a procedural vote.

The deal between Iran, the U.S. and five other global powers has divided the Democratic caucus, and lawmakers' concerns over its limitations left many grappling for weeks before reaching a decision.

"No deal is perfect, especially one negotiated with the Iranian regime," Ms. Mikulski wrote in a statement Wednesday. But she said her analysis left her convinced that the agreement "is the best option available to block Iran from having a nuclear bomb."

Democratic opposition to the agreement has come predominantly from New York and New Jersey lawmakers with large Jewish constituencies. Jewish groups have split over the deal, which critics said endangers Israel by releasing new resources that Iran may use to fund terrorist activities and that may leave Iran closer to developing a nuclear weapon.

Liberal groups and administration officials have pushed back, traveling often to Capitol Hill to sell lawmakers on the accord's merits and a lack of viable alternatives.

Even before the deal was reached, the White House had decided to focus its lobbying effort on Democrats, with Mr. Obama primarily reaching out to lawmakers from his own party. That early game-plan intensified in the days after the deal was announced, as Republicans voiced opposition to the deal and the White House became convinced the GOP would treat it as a political referendum on the president.

With Democratic momentum building for the deal in recent weeks, Democratic Sens. Bob Casey of Pennsylvania and Chris Coons of Delaware both said Tuesday they would support it.

"I'm very confident that all the pathways to a nuclear weapon are cut off," so long as the agreement is strictly enforced, Mr. Casey said in an interview Tuesday. Mr. Casey said he spoke to numerous White House officials, including several phone calls with the president, over the past few months leading up to his decision. He spoke to Secretary of State John Kerry as recently as last week.

"In the end, a persuasive case was made that there was no other option," Mr. Casey said.

It is not yet clear if opponents of the deal will be able to secure the 60 votes needed for the disapproval resolution to clear procedural hurdles in the Senate. No Republicans are expected to support the accord and two Senate Democrats also oppose it: Charles Schumer of New York and Bob Menendez of New Jersey.

The administration has been increasingly confident it may secure the 41 votes needed to block the measure's passage in the Senate, sparing the president the embarrassment of deploying a veto to safeguard the deal.

The White House plans to focus on maintaining the support Mr. Obama has for the deal and to continue lobbying lawmakers for additional backing. But with the announcement of the 34th Senate Democrat's support, the president can ease up on what has been an intense, seven-week campaign to gain enough votes to, at minimum, sustain his promised veto of any resolution opposing the deal.

In recent days as it appeared increasingly likely he would have the needed votes to sustain a veto, Mr. Obama softened his tone on what he believes would be the consequences if the agreement weren't implemented.

For weeks, Mr. Obama argued the only alternative to the July 14 agreement is military action and cast opponents of the deal as advocating for war against Iran. "Many of the same people who argued for the war in Iraq are now making the case against the Iran nuclear deal," Mr. Obama said in an Aug. 5 speech on the Iran deal. "The choice we face is ultimately between diplomacy or some form of war."

Yet Mr. Obama took a strikingly different approach last Friday when asked about his sharp rhetoric for critics of the deal.

"At no point have I ever suggested, for example, that somebody is a warmonger, meaning they want war," he said. "What I have said—and this I don't apologize for—is that if this deal is rejected…there has to be a better way to accomplish our goal of making sure Iran doesn't have a nuclear weapon."

Republicans still plan to fight to pass the resolution disapproval in both chambers, but have already begun formulating steps to keep pressure on Iran even if the nuclear accord begins peeling off sanctions.

"We've already seen Iran is willing to flaunt international rules before the ink dries," Sen. Cory Gardner (R., Colo.), a member of the Foreign Relations Committee, said in a recent interview. Mr. Gardner said he would support renewing and expanding sanctions on Iran for nonnuclear infractions or to punish any cheating on the deal.

Lawmakers return to Washington next week after their annual summer recess, and the Senate is expected to begin debating the deal next week. The timing of a House vote is still being determined.

Write to Kristina Peterson at kristina.peterson@wsj.com and Carol E. Lee at carol.lee@wsj.com

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(END) Dow Jones Newswires

September 02, 2015 12:35 ET (16:35 GMT)