Sen. Ben Cardin (D-Md.) will oppose the Iran deal. (Andrew Harnik/AP)

Sen. Benjamin L. Cardin (D-Md.) said Friday that he will vote against a landmark nuclear agreement with Iran, breaking with President Obama and the vast majority of his party over a pact that has been condemned by the Israeli government and key Jewish American leaders.

The agreement “legitimizes Iran’s nuclear program,” Cardin wrote in a Washington Post op-ed published Friday. He added that the pact offers “frighteningly quick relief from sanctions.” Imposing new sanctions if Iran violates the deal, he wrote, would “take too long to be effective.”

Cardin’s opposition is a symbolic victory for those who have fought to kill the deal; as the ranking Democrat on the Senate Foreign Relations Committee and an observant and prominent member of the Baltimore Jewish community, he was seen as an influential voice.

But the deal’s survival is already guaranteed. On Wednesday, Sen. Barbara A. Mikulski (D-Md.) became the crucial 34th vote Obama needed to secure a veto of any legislation opposing the pact.

Three more Democrats joined her Thursday, and a fourth joined Friday, helping Democrats move toward the 41 senators they would need to block any vote against the agreement through a filibuster.

Cardin is only the third Senate Democrat to oppose the pact, along with Sens. Charles E. Schumer of New York and Robert Menendez of New Jersey.

By announcing his position after the White House had already cobbled together enough yes votes, Cardin minimized the impact of his dissent on one of Obama’s signature foreign policy efforts.

In the op-ed, Cardin criticized the nuclear deal for referencing “good faith” and “mutual respect” between Iran and other nations, saying that “there cannot be respect for a country that actively foments regional instability, advocates for Israel’s destruction, kills the innocent and shouts, ‘Death to America.’ ”

The senator said he plans to introduce legislation that would establish as U.S. policy that Iran “will never be permitted to obtain a nuclear weapon and that all options remain on the table, including military options.”

His legislation also would make it easier to impose new sanctions for non-nuclear activity by the “rogue state,” he wrote.

Cardin has repeatedly pledged that his vote would be one of conscience rather than politics. He says he would strongly oppose any attempt to filibuster a vote on the agreement, arguing at a recent forum at the University of Maryland at College Park that every member of Congress should have the opportunity to register his or her position.

In making his decision, Cardin struggled with immense and personal pressure. His own rabbi repeatedly called Cardin — and his wife — to advocate against the agreement. The American Israel Public Affairs Committee held a rally at his synagogue. Supporters and opponents of the deal spent heavily on television ads in Baltimore and lobbied the senator’s staff.

Cardin is the last senator from the Maryland and Virginia delegations to reach a position, and he is the only one to oppose the agreement. Sen. Mark R. Warner (D-Va.), who also grappled with indecision for many weeks, came out in favor Thursday of what he called an “imperfect” deal.

“While I choose to support the deal, I am not satisfied with it as a final measure and will support efforts to shore-up its weaker points,” Warner said in a statement.

Warner said he was convinced that the five other countries involved in the agreement were not interested in renegotiating and would begin lifting sanctions with or without the United States.

Jenna Portnoy contributed to this report.