The debate over the proposed nuclear agreement with Iran will have its first prominent face-off today as Obama administration officials confront mostly skeptical members of the Senate Foreign Relations Committee.

Secretary of State John F. Kerry, Treasury Secretary Jacob J. Lew and Energy Secretary Ernest J. Moniz plan to argue to lawmakers that the agreement, if not perfect, is far better than any alternative.

Most Republican senators are hostile to the deal; several key Democrats, facing pressure from constituents and contributors, are uncommitted.

The administration is racing to win the support of enough lawmakers to defeat any resolution of disapproval at the end of the 60-day deliberation period Congress has given itself. At a minimum, it wants the support of 34 senators or 146 House members to prevent foes from overriding a promised veto by President Obama.

The three Cabinet secretaries briefed members of Congress on Wednesday in advance of the hearing. "This will make the region and the United States safer," Kerry said after an hourlong classified briefing Wednesday evening at the Capitol.

Sen. Richard Blumenthal of Connecticut, one of the Democrats who has yet to declare his position on the agreement, said he wanted assurances that the deal is "comprehensive, airtight, enduring and, most important -- verifiable."

Sen. Angus King, an independent from Maine who caucuses with the Democrats and is another important undecided vote, said that Wednesday's briefing was "reassuring."

"The important question starting to set in on people is: What are the alternatives?" King said. "The question is not is this a perfect agreement, but how does this agreement stack up against the alternatives? That was rather sobering to people in the room."

Critics of the deal have mounted an energetic lobbying campaign, with the goal of sinking the deal or at least badly weakening it before the Sept. 17 congressional deadline. If the agreement emerges from this summer's debate badly tarnished, it would lay the groundwork for repudiation by the next president, critics say.

"They'd like to see it on crutches" if not defeated entirely, said Cliff Kupchan, an Iran specialist and chairman of Eurasia Group, a political risk consultancy.

The Iran deal will lift U.S., European Union and United Nations sanctions against Iran, probably starting in about six months. In exchange, Iran must first roll back sensitive aspects of its nuclear program and agree to restrictions aimed at preventing it from obtaining a nuclear weapon.

Several issues are now dominating the debate.

One major dispute involves years 10 through 15 of the 15-year deal. Critics concede that the agreement might prevent Iran from getting a nuclear weapon over the next decade, but say it would allow Tehran to greatly expand its enrichment of nuclear fuel in the subsequent five years.

Administration officials acknowledge that during the latter years of the pact, Iran's "breakout" period -- the amount of time needed to obtain enough fuel for a bomb -- would gradually diminish from a full year because the country would be able to start using more advanced centrifuges. But other parts of the agreement would prevent the breakout time from falling precipitously, they say.

Critics also say the deal's procedures to verify Iran's compliance are too weak. In particular, they have focused on the fact that Iran could delay up to 24 days before allowing international inspectors access to sites where they believe cheating may be taking place.
Some experts, including Olli Heinonen, former deputy director of the International Atomic Energy Agency -- the U.N.'s nuclear watchdog -- contend that in that period, Iran could conceal evidence of important nuclear research, though it couldn't conceal traces of major projects such as construction of uranium enrichment plants.

Obama has defended that provision, noting that the facilities Iran would require to create a clandestine nuclear program would need to be extensive and hard to move.

"This is not something you hide in a closet. This is not something you put on a dolly and kind of wheel off somewhere," he said in a news conference last week.

Another question is whether the deal's mechanism to reimpose sanctions if Iran breaks its commitments is too cumbersome.

Skeptics contend that the world powers involved in the deal -- France, Britain, Germany, Russia and China, in addition to the U.S. -- will be reluctant to reimpose sanctions because doing so would threaten their carefully assembled deal and disrupt their countries' growing trade with Iran.

"While we've got sanctions snap-back, they've got nuclear snap-back," Sen. Bob Corker, the Foreign Relations Committee chairman, said after Wednesday's briefing. "In nine months, they'll have their cash and all the sanctions will be relieved. Then all the leverage sort of shifts to them."

A fourth big issue involves Iran's conventional weapons. Critics say that the deal's lifting of U.N. restrictions on Iran's purchase of arms after five years and missiles after seven and its freeing up of $100 billion to $150 billion in frozen Iranian assets will greatly strengthen the Tehran government.

The end of the arms embargoes coupled with additional money will allow Iran to step up purchases of arms that will strengthen itself and its proxies, such as Hezbollah, the Shiite Muslim militia in Lebanon, furthering inflaming violence in a region in upheaval, the deal�s critics charge.

But the administration and its allies say those embargoes were only in force because Iran had broken the nuclear rules and were always likely to end when it reached an agreement on nuclear issues.

The five- and seven-year extensions of the embargoes "we see as a bonus," said Kelsey Davenport of the Arms Control Assn., which supports the deal.

Administration officials also argue that most of the money resulting from the end of sanctions will be spent to bolster Iran's teetering economy. Officials contend that Iran's support for its proxies has been relatively cheap and hasn't been held back by a lack of cash.

The debate with lawmakers may also take a personal tone, since many members of Congress have suggested that Kerry and Obama caved to Iranian pressure because of their desire for a diplomatic triumph.

Kerry has been arguing in a series of interviews in recent days that he hung tough and almost walked out of the negotiations three times during the two-year duration of the talks.

This article was written by�Lisa Mascaro &�Paul Richter for Los Angeles Times on July 23, 205. Lisa Mascaro covers Congress in Washington, D.C. She writes about U.S. policy, economics and political culture. Paul Richter covers the State Department and foreign policy for the Los Angeles Times out of its Washington, D.C., bureau.