Lawmakers will begin a high-stakes and passionate debate when they return from a summer recess
US lawmakers will begin a high-stakes debate on a resolution disapproving the Iran nuclear deal when they return from a summer recess on Tuesday, ready to resolve an issue that stirred a passionate partisan division in Washington.
Although there is strong support for the resolution in the Republican-controlled Congress, President Barack Obama has said he will veto it and appears to have the Democratic votes needed to sustain the veto, ensuring the nuclear deal survives.
If enacted, a disapproval resolution would eliminate Obama's power to temporarily waive US sanctions on Iran passed by Congress, which analysts say are the majority of US sanctions, crippling the accord.
Here is a look at the Iran nuclear deal review process.
HOW THE REVIEW PROCESS WORKS
Obama signed the "Iran Nuclear Review Act of 2015" into law in May after congressional Republicans and Democrats demanded a chance to review any nuclear agreement with Iran.
The act allows Congress to consider a "resolution of disapproval" of the deal, announced on July 14, under which Iran agreed to curtail its nuclear program in exchange for relief from crippling economic sanctions.
Although a few Republicans have not yet announced their positions, enough already oppose it in both houses to pass the disapproval resolution. Three Senate Democrats and 14 House Democrats have said they will join Republicans in opposing the deal.
Republicans hold 54 seats in the 100-member Senate and 246 in the House, which has 434 members and one vacancy.
Congress has until Sept. 17 to pass the resolution. Obama then has 12 days to veto it and Congress has another 10 days to try to override his veto.
House leaders said they plan to vote on, and pass, a "resolution of disapproval" this week. The Senate is also expected to act quickly.
OBAMA CAN WIN WITH A VETO
Once the resolution is passed, and Obama issues his veto, both the House and Senate will need two-thirds majorities to override the veto. Obama thus needs either 34 votes in the Senate or 146 in the House to sustain his veto.
He already has promises from 38 senators - 36 Democrats and two independents.
Because the disapproval measure is a House resolution, the House will consider the veto first, so a Senate vote may not be needed.
By Friday, 110 House members, all Democrats, had expressed support for the nuclear pact, and House Democratic leader Nancy Pelosi has said she is confident she will muster enough votes to sustain a veto.
Deal supporters say forcing a veto would send a message to the world that the United States is not unified, weakening Obama just as he seeks to implement the deal.
Opponents say it strengthens the US position by telling the world the United States is skeptical and that the next president might back away, especially if Iran cheats.
HOW OBAMA COULD AVOID A VETO
If the Democrats can gather 41 votes in the Senate they could use procedural means - a filibuster - to stop the disapproval resolution from coming up for a vote.
But supporters said they have only slim hopes of mustering the 41.
No Republican has announced support for the agreement, and there are only five undecided Democratic senators. One, Joe Manchin, said he would not back a filibuster although he has not yet decided on the deal itself.
And some deal supporters may not back a filibuster, believing the measure should be allowed to come up for a vote.