US lawmakers vote Thursday on whether to advance a measure opposing the landmark deal that restricts Iran's nuclear
program, but Senate Democrats will almost certainly ensure US President Barack Obama's historic accord goes into force.
If the resolution is blocked, as expected, it would mark a huge victory for Obama and the most significant policy priority of his second presidential term.
The Senate holds a procedural vote at 3:45 pm (10:45 pm IST) on whether to end debate and move to a final vote on the resolution of disapproval of the deal, AFP reports.
A majority of lawmakers in the Republican-controlled Congress are opposed to the deal.
But the resolution requires 60 votes to advance in the 100-member Senate, and Obama's Democrats have more than enough to block the legislation from moving forward.
The July deal struck between Iran and six world powers provides Tehran relief from crippling economic sanctions in exchange for limits on its nuclear program.
Republicans complain the deal does not do away with the program altogether, fails to provide for spot inspections of nuclear sites or force Iran to end support for militant groups like Hamas.
"I ask colleagues to reflect on the gravely serious nature of the issue before us," Senate Majority Leader Mitch McConnell told the chamber as he pleaded with members to allow a final vote.
"This is a deal that will far outlast one administration," McConnell added. "The American people will remember where we stand today."
Senate Democratic leader Harry Reid acknowledged the "critical" national security aspects of the issue, but rebutted accusations that Democrats were trying to quash debate and block a final vote.
Senator Lindsey Graham, a Republican presidential contender and one of Obama's harshest foreign policy critics, issued a blistering warning to Democrats.
"I cannot believe you made the biggest miscalculation in modern history by empowering a religious fanatic with the ability to attack our nation, destroy our friends in Israel and keep the Mideast on fire for 15 years," Graham said. "What are you all thinking?
The House also votes Thursday on the first of three bills related to the Iran nuclear deal, after debate was delayed Wednesday amid a rank-and-file Republican revolt.
The House too was to vote on a disapproval resolution. But conservative lawmakers demanded a change of tactics, arguing that Obama violated the law by not providing the texts of secret so-called "side deals" to the accord.
Instead, the House will now vote on approving the Iran agreement, a move that forces Democrats to put their support for the accord on the record, which Republicans think may be more costly for Obama's party.