WASHINGTON — With just under a week left before the November 24 deadline on nuclear negotiations, high-level delegations arrived in Vienna Tuesday morning in a last effort to hammer out a deal that has been under negotiation for almost a year. The P5+1 negotiations are down to the wire, and, based on statements by negotiators, even the direction of a likely outcome is uncertain.

Resuming talks after a one-week hiatus, the round Tuesday was kicking off with a working lunch attended by Iranian Foreign Minister Javad Zarif and P5+1 lead negotiator Catherine Ashton, whose former position as EU high representative for foreign affairs and security policy officially ended on November 1.

Iranian news source PressTV suggested that the two wanted to use the lunch to “look into ways to continue the discussions,” but the parties were still far from clear that an extension of talks would be the outcome to the coming week’s diplomatic efforts.

The Obama administration sees, with varying levels of probability, three possible conclusions to this week’s marathon talks.

First – and perhaps most likely – talks could be extended. Washington has been working to make this option seem less appealing to its interlocutors. Congress, negotiators have emphasized, can only be kept on “good behavior” for so long. Any extension risks finding the US team operating under increased scrutiny from a Republican-controlled Congress that has already signaled willingness to pass tough new sanctions legislation that includes preconditions on the terms for any future agreement. If at times, the sense of urgency to reach the November 24 deadline seemed artificial or rhetorical, the Republican takeover of the Senate has given US negotiators a useful justification for a quick conclusion to talks.

The US has publicly indicated in recent days that it would be far preferable to reach a deal by November 24 than negotiate over the terms of an extension. At the same time, the administration reads the Joint Plan of Action as being extendable by mutual consent – not entirely ruling out the possibility of continued talks.

In the case of an extension, Washington warned last week, Iran will find itself working under less favorable terms. The same could be said — with the possibility of additional legislation opposed by the administration, passing both houses of Congress — for the administration itself.

The second outcome would be a comprehensive nuclear deal. The parameters of some parts of the deal seem clear, if only as delineated by the respective sides’ red lines. To Israel’s frustration, the P5+1 is likely to acquiesce to Iran’s claim that it has a right to domestic uranium enrichment. At that point, the negotiations focus around the number and type of permitted centrifuges, as well as the monitoring regime that will seek to provide advance warning of nuclear breakout. In this format, the comprehensive agreement — as the US has said — will seek to extend the window for breakout so that monitors will have a better chance of catching Iran in time if it tries to sprint for a nuclear bomb.

The US has indicated that its red lines lie across the Arak plutonium reactor. Administration officials have stressed that Iran may not enrich plutonium and that Arak must be either dismantled or significantly disabled. Last month, Russian Deputy Foreign Minister Sergey Ryabkov confirmed that “blocks of questions” remained regarding the reactor’s future.

A second US red line seems to be emerging around sanctions removal, with the US demanding that sanctions be removed gradually, and — to Iran’s frustration — only partially, leaving non-nuclear related sanctions in place. The Iranian state-sanctioned PressTV news agency cited Iranian sources close to the negotiating team as saying that the argument over removal of sanctions remains the major stumbling block to a deal.

If the Iranian report is accurate, sanctions relief could represent an intractable difference. US administration officials from both the State Department and the White House have emphasized – unprompted – in recent weeks that the sanctions relief on the table as part of the deal does not cover all of the American sanctions against Tehran. Instead, they have been clear both to the public and to hardliners in Congress that the only sanctions up for negotiation are the US-led sanctions stemming from Iran’s nuclear program. Other sanctions, including those that have been in place for three decades since the Iranian Revolution, and those that punish Iran for its human rights record, are not on the table, according to Washington.

According to a New York Times report, White House officials put the probability of reaching a deal in any case at 40 to 50 percent – hardly an optimistic projection for a major foreign policy initiative of this administration.

Even were a deal to be reached, both top US negotiator Wendy Sherman and National Security Council policy expert Phillip Gordon have emphasized that such an agreement would be far from representing a further warming of US ties with Tehran.

Gordon spoke of the possibility for a renewal of ties with the Islamic Republic as being cast on a generational timeframe, with a nuclear deal merely representing a sign of possibility for slow-moving, gradual normalization rather than a step in the process.

The third possible outcome is, of course, no deal at all. The Obama administration’s reading of the Joint Plan of Action could be defined as strict interpretationalist in this instance – reaching November 24 with no deal or plan to extend would mean that the sanctions relief that revived Iran’s economy in recent months would immediately be rescinded.

Foundation for Defense of Democracies Executive Director Mark Dubowitz expressed doubt last week that sanctions operated like a gauge that could be “dialed back” at will, citing months of economic development that have already made a tangible impact on Iran’s economy.

Backing away from a nuclear deal at the last minute would be far from an unprecedented step for Tehran. In 2009, Iran walked away from then-groundbreaking talks. Acting deputy secretary of state William Burns was the US point man on those talks, and is among the top US officials now in Vienna.

A hint toward the possibility that this week could see a repeat of 2009 could be found in the fact that US Secretary of State John Kerry, while slated to join his team in Vienna, had yet to finalize his arrival date in the Austrian capital as of Tuesday morning. Kerry is scheduled to spend an unspecified amount of time a short plane ride away in London, discussing Mideast policy, but seems to be waiting for some indication as to when would be the right time to join the Vienna talks together with his Iranian and EU counterparts.

Kerry’s arrival in Vienna could signal a last-ditch effort, or an incipient handshake. With the fate of Iran’s nuclear program – and a generation of geopolitics – at stake, a year’s worth of negotiations will come down to the diplomatic equivalent of a buzzer-beater three-point shot in Vienna.