WASHINGTON (AP) — Congress' vote on the international accord to curb Iran's nuclear program in exchange for billions of dollars in sanction relief stands as the most crucial national security decision since lawmakers voted in 2002 to authorize the invasion of Iraq.

Six weeks from a September vote, House Republicans formally introduced a resolution of disapproval on Tuesday. The GOP said Monday they have the 218 votes for the resolution, with Rep. Peter Roskam, R-Ill., calling the accord a "dangerous agreement."

President Barack Obama, meanwhile, is steadily securing the backing of Democrats crucial to ensuring the deal goes forward, with three senators announcing their support on Tuesday.

The lobbying is already intense, with a group backed by the pro-Israel group American Israel Public Affairs Committee investing in a multimillion-dollar ad campaign criticizing the deal. Another pro-Israel group, J Street, on Tuesday launched a television ad campaign as part of its $5 million campaign to rally support for the deal. Both sides are lobbying lawmakers.

Here is a news guide to the pact:

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REVIEW PERIOD

Congress is in the midst of a 60-day review of the historic deal and has until Sept. 17 to vote on a resolution either approving or disapproving the pact. During this time, Obama may not "waive, suspend, reduce, provide relief from, or otherwise limit the application of statutory sanctions with respect to Iran," which apply to the congressionally mandated penalties on various energy and banking sectors.

Congress returns from its August break on Sept. 8, and the Republican-led House and Senate are expected to vote for a resolution of disapproval and send it to Obama.

The House is expected to vote first since sanctions apply to money and under the Constitution, all revenue legislation starts in the House.

The president will have 12 days to veto the measure, as he has promised. The House and Senate then would have 10 days to vote on overriding a veto.

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IF CONGRESS DISAPPROVES OF THE DEAL, THE VOTES OBAMA WOULD NEED TO SUSTAIN A VETO

In the 434-member House, one-third plus one of those voting. There is one vacancy in the House — former Rep. Aaron Schock's seat in a Republican-leaning district. It will be filled in a special election on Sept. 10.

In the 100-member Senate, one-third plus one of those voting.

Only one chamber of Congress is needed to sustain a veto.

Republicans uniformly oppose the deal. The Obama administration is counting on Democrats to stand with the president and preserve the pact. Obama, whose relationship with congressional Democrats has sometimes been rocky, has gotten high marks from his party for the administration's outreach, including Capitol Hill briefings and lengthy sessions at the White House.

House Democratic leader Nancy Pelosi, who in 2010 secured the votes for Obama's first-term priority of overhauling health care, has said there are a sufficient number of Democratic votes in the House to sustain a veto for the president's second-term priority of an historic deal.

The White House also expressed confidence on Monday that Obama's veto would be sustained in the House though the administration was still pressing its case with lawmakers.

So far this week, five Democrats have announced their support of the deal: Sens. Barbara Boxer of California, Bill Nelson of Florida and Tim Kaine of Virginia backed it on Tuesday. Rep. Adam Schiff of California, the top Democrat on the House Intelligence committee; and fellow California Democratic Rep. Anna Eshoo, announced their support on Monday.

Freshman Democrat Kathleen Rice of New York announced her opposition.

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THIS WEEK

Obama was meeting with American Jewish leaders on Tuesday and delivering a speech on Wednesday on the Iran deal.

Ambassadors from the Western powers that signed onto the deal also met privately with Senate Democrats on Tuesday.

Yukiya Amano, director general of the International Atomic Energy Agency, on Wednesday is to brief members of the Senate Foreign Relations Committee, which also is holding two hearings to hear from experts.

Wendy Sherman, undersecretary of state for political affairs who helped negotiate the deal, and Adam Szubin, acting undersecretary of the Treasury for terrorism and financial crimes, testify on Wednesday before the Senate Banking Committee.

In another sign of the administration's all-out campaign, Sherman, Szubin and Energy Secretary Ernest Moniz also will brief senators on Wednesday in a closed-door session.