With a framework deal to halt Tehran's nuclear program, Obama moved closer to the kind of staggering diplomatic breakthrough with the Islamic Republic that would have been unthinkable just a few years ago.

If the political agreement reached in Switzerland turns into a genuine pact honored by both sides, Obama will be entitled to a place in history as the leader who defused an intensely bitter estrangement with Iran.

But he also took personal ownership of a fraught negotiating process full of false starts and deep divisions, one that hinges on the sides' ability to hammer out a host of devilish details by a June 30 final deadline in the face of vocal opposition from domestic and international critics.

If the deal falls apart, it will be hard to refute charges by critics that Obama's insistence on negotiating directly with U.S. enemies -- a tactic at the heart of his political philosophy -- is deeply naive and futile.

The risks of Obama's choice, and the challenge of resolving tough issues to get to a final agreement by July, were clear within minutes of news breaking that a deal was reached in Lausanne.

Obama quickly appeared in the White House Rose Garden, not for the victory lap that presidents often take in this picturesque spot, but to launch an impassioned defense of the contentious deal.

His sales pitch was concise: There is no other better way to prevent Iran from moving covertly to build a nuclear weapon.

"When you hear the inevitable critics of the deal sound off, ask them a simple question: Do you really think that this verifiable deal, if fully implemented, backed by the world's major powers, is a worse option than the risk of another war in the Middle East?" Obama said.

"Is it worse than doing what we've done for almost two decades with Iran moving forward with its nuclear program and without robust inspections?"

The question now is whether Obama's skills of persuasion -- hardly his strong suit -- will convince critics that his negotiators got a good deal. First signs were not encouraging for the White House.

Republican House Speaker John Boehner warned that Congress would continue to press for a vote on the deal, which might derail its long-term prospects given the extent of Republican opposition. Another pending bill that has the potential to scuttle the negotiations would impose additional sanctions on Tehran.

Republican Sen. Lindsey Graham of South Carolina attacked the president's posture on Thursday as he, too, emphasized that Congress must review a final deal.

"We simply cannot take President Obama's word that it is this or war," he said.

The March 31 deadline -- twice pushed back -- was originally imposed on the process in order to help Obama's political prospects of selling the deal to Congress, which has final say on lifting U.S. sanctions on Iran. Several Democrats had indicated that they planned to join with Republicans on the controversial bills, but they pledged to hold up consideration of the measures until late March so Obama could show the talks were making progress and should be bolstered rather than tanked by legislators.

While the Republicans' response Thursday demonstrated that the framework deal had not placated them, skeptical Democrats were more noncommittal on how they would respond.

"We now need to take a close look at the details to determine if the compromises made are worth the dismantling of years of pressure built on Iran," said Rep. Eliot Engel of New York, the top Democrat on the House Foreign Affairs Committee.

New York Rep. Steve Israel more clearly showed that Obama could well face an intraparty challenge.

"The details deserve and must get a vote by the U.S. Congress," he said in a statement. "Until the full details are provided to Congress on June 30th, you can keep me in the 'highly skeptical' column."

Obama also faces intense displeasure from many of America's closest allies in the Middle East, countries such as Israel and Saudi Arabia that are directly in the Iranian line of fire. They are concerned that the United States may be giving up leverage on Iran by lifting sanctions while leaving Tehran's nuclear infrastructure intact.

The president nodded to this challenge in the Rose Garden when he said that he would invite the leaders of the Gulf Cooperation Council to Camp David this spring to discuss raging Middle East turmoil, much of it aided by Tehran.

He also spoke to Saudi Arabia's King Salman and Israeli Prime Minister Benjamin Netanyahu, with whom he has feuded over Iran.

Netanyahu has powerful allies on Capitol Hill, and will be sure to lobby for the bills seeking to constrain the administration in its deal-making with Iran as the final deadline nears.

Already Thursday, Israel called the celebrations in Switzerland "disconnected from reality" and said Iran would use a "poor framework" for a "bad and dangerous" deal to move towards nuclear war.

And the Obama administration's Iranian counterparts have their own treacherous path to getting approval of their part of the deal -- making Obama's bold endorsement of the provisional agreement particularly perilous.

Iran's top negotiator, Foreign Minister Mohammad Javed Zarif, must convince Supreme Leader Ayatollah Ali Khamenei and hardliners in Iran's Revolutionary Guard Corps to accept the deal and permit its implementation.

"Javad Zarif will have to sell this deal like we will. His task is not simple, or a given," a senior administration official said.

Like Obama, Zarif wasted no time, boasting at a news conference in Switzerland that Tehran had retained its right to enrich uranium (to 3.67 percent, according to a White House fact sheet distributed at the deal's announcement) and would not lose its nuclear infrastructure.

And Iranian swagger like that -- a political necessity for Zarif -- emphasizes the very aspects of the deal that make its American critics most concerned.

Another key point of contention is how comprehensive the inspections will be. While the U.N.'s International Atomic Energy Agency will have unprecedented access to Iran's declared nuclear facilities under the deal, many in the West wanted inspectors to have unfettered access to any site of their choosing since Tehran has hidden nuclear operations in the past.

The White House fact sheet said that the IAEA inspectors "will have regular access" to all of Iran's facilities but did not specify how that would be achieved.

"The nuclear flaw in this agreement is the fact that we will not be able to go anywhere, go anytime," said Mark Dubowitz of the Foundation for the Defense of Democracies.

"The IAEA is going to have to work with the Iranians. What the Iranian government has shown over decades is the ability to defeat the IAEA with stonewalling, delay and deviousness."

But the senior official said that the U.S. negotiating team was confident that the talks on a final deal would produce an agreement on a "mechanism" that would resolve disputes over access to Iranian sites.

Skeptics also questioned Obama's assurance that lifted sanctions could "snap back" in place if Iran transgressed once the agreement went into force. Debates are already raging about the sequence in which sanctions will be lifted on Iran and on why the United States would bolster Iran's coffers by lifting sanctions at a time when it is blaming Tehran for destabilizing the Middle East.

The White House, however, has other concessions to point to.

The deal will cut Iran's stocks of centrifuges, require the conversion of an underground enrichment facility at Fordow to a research center and limit the output of another reactor at Arak, among other requirements.

In return the United States and other world powers will lift sanctions that have throttled Iran's economy, offering it the tantalizing prospect of a return to full membership within the international community.

Karim Sadjadpour, an Iran expert at the Carnegie Endowment for International Peace, said the deal was close to "win, win" for both Iran and the United States. He added, however, "We don't want to get too ahead of ourselves."

He continued, "What was announced today is the engagement. The wedding is scheduled to take place in July, but there is going to be a vigorous debate about the prenuptial agreement, and there is no guarantee this wedding will take place on time."

Though the four-page White House fact sheet left many technical questions unanswered, the deal surprised some experts and political figures with its detail and specificity.

That's something that Jim Walsh, from the Security Studies Program at Massachusetts Institute of Technology, suggested could help sway worried Democrats.

"I think they have put themselves in pretty good shape to go to that community and defend the deal. They ended up with a lot more than most of us were expecting," Walsh said.

Still, even Obama admitted that the success of the initiative was far from certain. If the framework deal snags on the unresolved technical details before the final deadline on June 30, or if Tehran tries to cheat in years to come, Obama's hopes of a foreign policy victory for the ages will founder as well.

"The President's strategy has been absolutely incoherent in the Middle East in general. He is pinning his legacy on this agreement," Republican Rep. Martha McSally of Arizona told CNN's Wolf Blitzer.

But so far, that legacy has received a boost from the week's events, even though Republicans tried to paint the twice-delayed announcement of the deal as a sign that Obama wouldn't be able to deliver. That doesn't mean, though, that his fortunes couldn't change -- and change quickly.

"You've got quite a significant accomplishment," Aaron David Miller, a former U.S. Middle East peace negotiator, told CNN. "Is it perfect? No."