The expected showdown in Congress next month over the administration's nuclear deal with Iran is only the first in a series of challenges that could leave the fate of the historic agreement uncertain for years and blemish President Obama's most ambitious foreign policy effort.

Although the accord appears on track to survive congressional opposition in the short term, if necessary with a presidential veto, opponents have begun laying plans to force its overhaul or repudiation down the road. Republican presidential candidates are united against it, and several have vowed to discard or amend it if elected.

Even if it overcomes those obstacles, the deal could collapse during implementation if clear evidence emerges of Iranian cheating, if major disputes erupt over inspections and rules, or if governments and United Nations agencies fail to properly oversee it.

The agreement also could be undermined by Iranian hard-liners who see a threat to their business interests, or by friction with Iran over the civil war in Syria or other Middle East conflicts.

"There's always the risk of some snafu, or some latent issue, even if the initial implementation goes well," said Richard Nephew, a former U.S. negotiator who supports the deal.

Aware of the dangers, the administration plans to name a senior diplomat, backed by officials from the Treasury and Energy departments, to oversee the initial implementation phase. The appointment is intended to ensure that the rollout gets sufficient attention before Obama leaves office in 2017.

The complex deal, reached between Iran and the six world powers in Vienna on July 14 and later affirmed by the U.N. Security Council, seeks to curb Iran's nuclear activities for at least a decade in exchange for easing of international economic sanctions.

Administration officials say Iran could regain access by mid-2016 to about $55 billion frozen in overseas accounts if it meets its obligations to destroy, suspend or scale back production of nuclear fuel and other activities needed to build a bomb. Iran denies that it is seeking a nuclear weapon.

Obama's supporters consider the deal the president's foremost foreign policy achievement and one of the most important diplomatic accomplishments in decades. Critics argue that it will quickly benefit Iran's government by lifting sanctions and allow it to again enrich uranium at high levels, if it so chooses, in 10 to 15 years.

Congress faces a self-imposed Sept. 17 deadline to approve or reject the deal. It's not yet clear whether Senate opponents will get the 60 votes they would need to break a filibuster and advance a resolution designed to kill the accord. Even if Congress did so, the White House is confident that opponents cannot muster the two-thirds majority needed in both houses to override a presidential veto.

Privately, some senior congressional officials and leaders of advocacy groups who oppose the deal acknowledge that they are likely to fall short and that it will survive, giving the White House a green light to start implementation in coming months.

Yet opponents are expected to portray that loss as a win. A strong congressional vote to reject the accord, even if the effort fails, will signal presidential weakness and tarnish the deal's luster, they argue.

Critics say that if enough Senate Democrats join the 54 Republicans and boost the no vote to the low 60s � enough to clear the filibuster hurdle and pass the resolution with a simple majority, but not the 67 needed to overcome a veto � it would send a strong message for the next president to push for changes in the deal and to give Iran no leeway in implementation.

Senate Minority Leader Harry Reid of Nevada announced his support Sunday, the 27th Senate Democrat to back the deal. Only two, Charles E. Schumer of New York and Robert Menendez of New Jersey, have come out in opposition. Republicans would need 13 Senate Democrats to overturn a veto.

Obama's "nightmare scenario is not that the deal is blocked, but that he's handed a resounding political defeat" and the accord is undermined, said Mark Dubowitz, executive director of Foundation for Defense of Democracies, one of the most active opponents of the accord.

In theory, that could convince international companies that American support is uncertain and they could face new U.S. sanctions if they invest or do business in the Islamic Republic. That resistance could, in turn, weaken Iranian support for the agreement.

Administration officials say the next president, who won't take office until 2017, is likely to stick with the deal if the initial implementation goes relatively smoothly. A new administration would risk international condemnation if the accord collapsed and Iran resumed enriching uranium, they argue.

"If you break the deal, and you don't have a better alternative for how to control Iran's nuclear program, people will say, 'Why did you do that?'" said Nephew, now a program director at Columbia University's Center on Global Energy Policy. "Killing the deal's much harder when you're not a candidate, but in charge."

Even so, the next president might use minor Iranian infractions as a pretext to pull out. Or demand such harsh penalties for the infractions that Tehran withdraws its support. Simply ignoring the deal could be enough to undermine it.

"I worry that in the next administration, they don't give it the same level of priority and over time it starts to fall apart," said Ilan Goldenberg, a former Obama administration official now with the Center for a New American Security think tank.

Another potential threat comes from future legislation aimed at Iran.

Many lawmakers and advocacy groups have vowed to hit Iran with new penalties for human rights abuses, support for terrorism, regional aggression and other disputed activities. The administration says new sanctions for such purposes don't violate the deal and may be justified.

"The United States should ratchet up the costs to Iran for its oppressive policies and regional meddling," Jonathan Greenblatt, national director of the Anti-Defamation League, which opposes the deal, wrote on the group's website.

Dylan Williams, legislative director of the pro-Israel group J Street, which favors the agreement, predicts that it will face legislation in Congress that "while claiming to target things like Iranian terror and human rights abuses, would deprive Iran of most of the economic benefit it would get" under the agreement.

The nuclear deal "could become the Obamacare of foreign policy," targeted year after year by foes who want to bring it down, he said.

This article was written by Paul Richter for Los Angeles Times on Aug. 24, 2015. Paul Richter covers the State Department and foreign policy for the Los Angeles Times out of its Washington, D.C., bureau.