The Obama administration has said consistently for two years that it wants to reach an agreement that ensures Iran's nuclear program remains peaceful and can never be used to develop a nuclear weapon.

But the administration has made a series of concessions, almost from the start, that have the potential to thwart the basic goal of the talks, even if an agreement is reached. Those concessions have contributed to a wide gap in public opinion, with majorities of voters telling pollsters they support the idea of reaching a deal but are also skeptical that any agreement will meet its goal. This view is reflected in the concerns of a bipartisan majority of lawmakers.

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"The administration's deadlines and redlines with Iran are all moving in the wrong direction, and the backpedaling is a major threat to our security. Achieving a nuclear deal at all costs is not only short-sighted, it is dangerous," House Armed Services Chairman Mac Thornberry, R-Texas, said Wednesday.

Here are five of the most important concessions and why they may thwart the deal:

1. Enrichment: U.N. Security Council resolutions bar Iran from enriching uranium, and the Obama administration quickly brushed off Tehran's assertion that a November 2013 interim agreement recognized it's "right" to enrich by allowing limited enrichment activities to continue.

But international negotiators from the five permanent U.N. Security Council members and Germany have backed down on that point. The framework for a final deal announced April 2 recognizes that Iran will continue to enrich uranium.

The question of enrichment is one of the top concerns for neutral experts looking at a potential deal, since it's not only a backdown from longstanding U.S. nonproliferation policy, but also rewards Iran for years of defying the Security Council on this point. And as Iran's knowledge of the process grows, it will be harder to keep it from being able to produce bomb-grade uranium. Indeed, many experts, and even Secretary of State John Kerry, have acknowledged that Tehran may already have that ability if its program is left unchecked.

"Iran has zero need for enrichment, yet it gets to enrich in a region of tension after violating numerous arrangements," David Albright, founder of the Institute for Science and International Security, told the Senate Foreign Relations Committee last week. He suggested that lawmakers evaluating the deal make clear that the United States continues to reject the idea that Iran has a right to enrich.

2. Time limits: The April 2 framework sets limits on Iran's enrichment abilities and stockpiles of enriched uranium lasting from 10-15 years. This is longer than what Iran had reportedly asked for, but not the permanent arrangement administration officials had indicated the final deal would be.

The idea behind the time limits is to keep Iran at least a year away from having enough enriched uranium for at least one nuclear weapon during that period, since any final deal would no longer require the destruction of much of the country's nuclear infrastructure as initially expected when the talks began. But in an April 7 interview with NPR, President Obama admitted that once those time limits were lifted, that "breakout" period could disappear.

"What is a more relevant fear would be that in year 13, 14, 15, they have advanced centrifuges that enrich uranium fairly rapidly, and at that point the breakout times would have shrunk almost down to zero," he said.

3. Verification: The question of what access inspectors have to Iranian nuclear sites, especially military ones, along with enforcing the Security Council's requirement for Iran to account for its past work on nuclear weapons, are the two biggest obstacles to getting a deal at the current talks in Vienna, the New York Times reported Wednesday.

International Atomic Energy Agency chief Yukiya Amano was set to meet Thursday with Iranian President Hassan Rouhani in a bid to resolve those questions, which also have complicated the Obama administration's ability to sell a deal amid reports U.S. negotiators were backing off previous hardline stances on them.

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Meanwhile, Ayatollah Ali Khamenei, Iran's supreme leader, has insisted that international inspectors could not have access to military sites, and Iran has yet to fully cooperate with the IAEA's requests for information on past nuclear work.

Arms control experts agree that it would be impossible to verify that Iran's nuclear program is peaceful if an agreement limits access to international inspectors, or lets Iran get away without a full accounting of past work.

"Iran's intransigence on that point is very disturbing," Albright told senators.

Kerry sparked a backlash in Congress last month when he confirmed news reports that negotiators would not demand a full accounting of Iran's past work on nuclear weapons as a condition of any agreement because "we have absolute knowledge with respect to the certain military activities they were engaged in."

Obama, however, said on Tuesday that if the verification regime was inadequate "then we're not going to get a deal, and we've been very clear to the Iranian government about that."

4. Sanctions relief: It's clear that Iran will get relief from international sanctions in exchange for agreeing to limits on its nuclear program, including the release of more than $100 billion in frozen assets. That's the basic exchange underpinning any deal.

But the question of when that relief will come is still under negotiation, and, like the verification issue, could prevent negotiators from reaching agreement at all. Iranian officials, backed by Khamenei's demand for the "immediate removal of economic, financial and banking sanctions" once any nuclear agreement takes effect, are resisting a phased implementation of relief.

It would become a serious issue for Congress in approving any deal if the administration agrees to Khamenei's condition, since most U.S. sanctions in that area were imposed because of Iran's continuing support for terrorism, not the nuclear issue. Obama and Kerry have repeatedly promised lawmakers they would lift only nuclear-related sanctions as part of any deal.

5. Fordow and Arak: The administration has insisted that the heavily fortified underground research site at Fordow and the heavy-water reactor at Arak be closed. Under the April 2 framework, both will remain open, and Iran will continue to be able to run centrifuges at Fordow, but not to enrich uranium until after 15 years. The core of Arak's reactor is to be replaced with a new core based on a design approved by international negotiators, and spent fuel is to be shipped out of the country.

Allowing Iran to keep both facilities is a concession that experts say not only preserves key infrastructure, but also means they could be used to build a nuclear weapon. Fordow's location, which is deep in a mountain, would protect that effort from military retaliation if Iran decided to break out.