In an agreement that he called "a long time coming," President Obama announced that the U.S., Iran and other countries have reached a historic framework to curb Iran's nuclear program. (AP)

With the end to hard-fought negotiations over Iran’s nuclear program in sight, the Obama administration must now sell the deal to what may be far more formidable adversaries in Congress, and to skeptical allies, including Israel, in the Middle East.

President Obama began that process Thursday immediately after the framework agreement was announced, with calls to Saudi Arabia’s King Salman and Israeli Prime Minister Benjamin Netanyahu. He invited Persian Gulf leaders to meet with him this spring at Camp David, while the last details of the accord are being nailed down in anticipation of a final deadline in June.

He also began a series of calls to congressional leaders and promised his administration would work with lawmakers on how they can play what he called a “constructive oversight role.”

But even as he promised close consultations across the board, Obama made clear that he would hold opponents responsible if they tried to undercut a deal whose importance he compared to historic arms-control agreements of the past and which he considers a crucial part of his legacy in office.

“The issues at stake here are bigger than politics,” he said. “These are matters of war and peace.”

The Post's Dan Balz talks to Yahoo News about the nuclear framework agreement reached between Iran and world powers, and the political ramifications for President Obama. (Yahoo News)

“If Congress kills this deal — not based on expert analysis, and without offering any reasonable alternative — then it’s the United States that will be blamed for the failure of diplomacy,” Obama said. “International unity will collapse, and the path to conflict will widen.”

The warnings came at the end of a lengthy Rose Garden statement in which Obama outlined the parameters of what he called “a good deal” that “would cut off every pathway that Iran could take to develop a nuclear weapon.”

Even the negotiators in Lausanne, Switzerland, seemed surprised at how far they had come. After more than a year of meetings, the last several days of round-the-clock sessions took them to the heights of optimism, only to see “our hopes crash” when it appeared that “maybe we just can’t get there,” said a senior administration official, one of several who briefed reporters on the condition of anonymity because of ground rules imposed by the White House.

The final 24 hours, by all descriptions, were intense. Secretary of State John F. Kerry, leading the U.S. negotiating team in Lausanne, told Obama “how close they were” in a telephone call Wednesday afternoon, and national security adviser Susan Rice updated the president at midnight. At 10 a.m. Thursday, Obama was told the parameters of the deal reached during all-night meetings, and “at that point, essentially, he signed off,” the senior official said.

Obama placed calls to the leaders of Britain, France and Germany who, along with Russia and China, joined forces across the table from Iran.

Whatever satisfaction the administration may have felt, attention quickly turned to the next phase. Officials reiterated their belief that Congress has no formal role in approving a final deal and vowed to head off any congressional attempt to vote on it, especially before it is due to be completed and signed three months from now.

The fear is that a vote opposing the deal would allow Iranian hard-liners to argue that the United States may not be able to hold up its end of the bargain.

Highlights of the Iran deal

Principal among several bills pending is one, due to be taken up the day Congress returns from recess on April 14, that would withhold sanctions relief for Iran under the final deal until lawmakers had a 60-day period to review and vote on it.

A U.S. fact sheet is somewhat vague on the timetable for lifting nuclear-related U.S. and international sanctions on Iran, saying they will be suspended after international monitors “verified that Iran has taken all of its key nuclear-related steps.” The sanctions could “snap back into place” if Iran “fails to fulfill its commitments.”

“We think it’s best for members of Congress to take a look at the framework and give the space to negotiate . . . between now and June,” an administration official said.

Most lawmakers reacted cautiously to the framework announcement. Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.), who authored the 60-day review bill, said it was “important that we wait to see the specific details” of the framework, but that he still planned to bring the measure before his committee when Congress returns from its Easter recess.

With eight Democrats also sponsoring the bill, the administration has been worried that the measure could win a veto-proof majority. Sen. Tim Kaine (Va.), one of the Democratic sponsors, appeared to take both sides of the argument, calling the framework “a positive step” that “if fully implemented, would protect the world from nuclear proliferation in the region.” But he also said he would “continue my efforts to build bipartisan support” for the bill.

Sen. Bill Nelson (Fla.), another of Corker’s Democratic sponsors, made no mention of the legislation, however, and said in a statement that he was “cautiously optimistic” that the agreement “just might work.”

The most outspoken opponents of an agreement appeared unimpressed by the new framework details. “There is no nuclear deal or framework with Iran; there is only a list of dangerous U.S. concessions that will put Iran on the path to nuclear weapons,” Sen. Tom Cotton (R-Ark.) said in a statement.

The administration’s fact sheet laid out four pages of specifics, including a two-thirds reduction in the number of Iran’s centrifuges and strict limits on the amount and degree of enriched uranium they can process. It calls for repurposing Iran’s once-secret underground enrichment facility at Fordow into a research lab, and rebuilding its Arak heavy-water reactor so that it can no longer produce plutonium, which can also be used in nuclear weapons. Timelines on the various restrictions and a strict international inspection regime go from 10 years to forever.

Other elements that remain vague, and that are likely to draw concern from lawmakers, include outstanding questions about military elements of Iran’s past nuclear programs, and specifics on when Iran would be released from enrichment restrictions. Critics are likely to argue that mothballing, rather than destroying, centrifuges leaves too many options open.

Many nuclear experts said the outlined agreement was better than they had anticipated. “If the Iranians actually agree, it’s pretty impressive,” said George Perkovich of the Carnegie Endowment for International Peace.

Alireza Nader, a Rand Corp. senior policy analyst, agreed that the parameters of the agreement were “quite strong, and may even exceed the expectations of some critics. Iran appears to have made significant concessions, especially on the number of centrifuges and the verification . . . regime.”

But while the administration emphasized the unprecedented restrictions and inspections the deal entailed, Iranian officials catalogued what they considered victories.

“None of these measures include closing nuclear facilities,” Iranian Foreign Minister Mohammad Javad Zarif said at a news conference in Lausanne. “We will continue enriching . . . research and development. Our heavy-water reactor [at Arak] will be modernized. . . . The effect of it will be, when we implement our measures, there will be no sanctions against the Islamic Republic of Iran.”

“It’s a win-win outcome,” Zarif said.

Mike DeBonis, Robert Costa and Paul Kane contributed to this report.