Washington (CNN) President Barack Obama believes the chances of getting a nuclear agreement with Iran are now "below 50-50," according to a top Democratic senator who attended a closed-door meeting with the President Tuesday night.

"He said in the course of the negotiations he's been more optimistic, less optimistic. And he said that the chances at this point are below 50-50," said Sen. Dick Durbin of Illinois, the Assistant Democratic Leader and a close ally of Obama's.

The comments lowering expectations for a deal curbing Iran's nuclear program come after negotiations on a final deal have already blown through two deadlines and seem likely to go past a third on Friday.

Another lawmaker briefed on the negotiations explained to CNN that the administration has been taken aback by last-minute Iranian refusals to give ground on revealing the past military dimensions of the program and their insistence on lifting missile restrictions.

In a last-minute push to reach a deal by Friday, the duration of the deal and efforts to enshrine any deal into a UN Security Council resolution have become serious sticking points. In the resolution, Iran has demanded a lifting of a UN arms embargo, which world powers are currently reluctant to do.

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey spoke against any such easing in particularly categorical terms on Capitol Hill Tuesday.

"Under no circumstances should we relieve pressure on Iran relative to ballistic missile capabilities and arms trafficking," he told a panel of lawmakers.

Sources involved with the negotiations said red lines laid out last month by Iran's Supreme Leader, Ayatollah Ali Khamenei, have complicated the talks. The demands, delivered in a speech and reposted on Twitter, included a shorter duration for a deal and its restrictions on Iranian nuclear research, immediate removal of economic sanctions once a deal is hatched and the banning of inspections at Iranian military sites. All seemed contrary to the political framework agreed to in April in Lausanne, Switzerland.

They sources said, however, that progress has been made on answering lingering questions about Iran's weapons program, and that gaps have also been narrowed on other thorny issues, such as the pace and scope of sanctions relief and defining limits on the type of research and development Iran will be permitted to do while the deal is in effect.

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"There is no last-minute monkey wrench though," one Obama administration official said. "We are seeing pretty standard Iranian tactics that we were ready for."

Still, the sources would not predict whether a deal would be done by Friday. Most of the issues, the sources said, will take high-level political decisions by Iranian leadership that they may not be willing to make this week.

"If Iran made the decision and said, 'fine, we have a deal,' we could have a deal today and finish up writing the agreement in a couple of days," a senior administration official said. "But they may need to show that (they) are willing to walk away for a while and come back later. They also may need to go back one more time to the Supreme Leader."

Durbin noted that Obama made his comments before getting a daily update from Secretary of State John Kerry, who is spearheading the Vienna-based nuclear talks for the U.S.

"I think it's an indication it's crunch time. He said he's not going to accept a weak or bad deal. He knows what's at stake here," Durbin explained.

One diplomat pointed out, "It is possible (that) there comes a limit, where everyone needs to go home, regroup and think about things and come back to finish. But nobody is in the market for long extensions here."

Photos: Iran: Key players in the nuclear talks Photos: Iran: Key players in the nuclear talks After arduous talks that spanned 20 months, negotiators reached a landmark deal aimed at reining in Iran's nuclear program, announced on July 14. From left, European Union High Representative for Foreign Affairs and Security Policy Federica Mogherini, Iranian Foreign Minister Mohammad Javad Zarif, Head of the Iranian Atomic Energy Organization Ali Akbar Salehi, Russian Foreign Minister Sergey Lavrov, British Foreign Secretary Philip Hammond and U.S. Secretary of State John Kerry pose for a group picture at the United Nations building in Vienna on July 14. Hide Caption 1 of 10 Photos: Iran: Key players in the nuclear talks U.S. Secretary of State John Kerry has been spearheading negotiations on a possible deal to rein in Iran's nuclear program. Hide Caption 2 of 10 Photos: Iran: Key players in the nuclear talks Kerry, second from left, meets Iranian Foreign Minister Mohammad Javad Zarif, second from right, for talks in Lausanne, Switzerland, on Monday, March 16. At the far left is U.S. Secretary of Energy Ernest Moniz. At the far right is Ali Akbar Salehi, head of Iran's Atomic Energy Organization. Hide Caption 3 of 10 Photos: Iran: Key players in the nuclear talks Iranian Supreme Leader Ayatollah Ali Khamenei meets with environmental officials in Tehran, Iran, on Sunday, March 8. Hide Caption 4 of 10 Photos: Iran: Key players in the nuclear talks Iranian President Hassan Rouhani speaks in September during a news conference in New York. Hide Caption 5 of 10 Photos: Iran: Key players in the nuclear talks Wendy Sherman has been a key U.S. negotiator in the Iran talks. She is the under secretary of state for political affairs. Hide Caption 6 of 10 Photos: Iran: Key players in the nuclear talks Federica Mogherini, foreign policy chief for the European Union, has been representing the Europeans in nuclear talks with Iran. Hide Caption 7 of 10 Photos: Iran: Key players in the nuclear talks Iran's parliament speaker Ali Larijani listens to a question during a news conference in Tehran on March 16. Hide Caption 8 of 10 Photos: Iran: Key players in the nuclear talks Yukiya Amano is director general of the International Atomic Energy Agency, the U.N.'s nuclear watchdog. Hide Caption 9 of 10 Photos: Iran: Key players in the nuclear talks Iran and world powers comprising the P5+1 negotiating team meet in Montreux, Switzerland, on Thursday, March 5. Hide Caption 10 of 10

For now, though, diplomats said they continue to make progress, though one Western diplomat acknowledged, "There will come a point when we are not making progress and then it will be time to come home."

But the sources said that nobody in the six-power negotiating group, known as the P5+1, is concerned about the optics of the prolonged negotiation, and they will stay as long as the negotiations are productive and gaps are being narrowed.

In an op-ed published Wednesday in the Financial Times, Iranian Foreign Minister Mohammad Javad Zarif wrote, "Getting to 'yes' necessitates the courage to take the higher ground, sufficient fortitude to be flexible, the audacity to shatter old habits, and most of all, a vision for a better future."

Sources said that Iranian negotiators seem to have an inflated expectation of the time and ease by which they can take nuclear steps called for in the deal before sanctions can be lifted.

"Our biggest concern is not that they aren't serious about this," one senior administration official said. "They think it will be easy for them to do what they have to do. These things called for in the deal will take time. It will be hard for them technically."

When asked how negotiators would know whether to walk away or stay at the table, a senior administration official briefing reporters late Tuesday said, "you know when that moment comes."

"You always get to a place where you're at a precipice," the official said. "You're either going to pull back from the precipice, or you're going to go over the cliff."