The Iran nuclear deal strongly backed by President Barack Obama and just as ardently opposed by the Republican Party is now on track to get through Congress.
In the Senate, Democratic supporters now claim a decisive 34 votes in favour, after Sen. Barbara Mikulski of Maryland called the pact "the best option available to block Iran from having a nuclear bomb."
That will allow backers to uphold Obama's veto, if necessary, of a resolution of disapproval Republicans are trying to pass this month. Republican lawmakers who control the House and Senate say Iran got too many concessions in the agreement, which aims to curb the country's nuclear program in exchange for hundreds of billions of dollars in relief from international sanctions.
Republican Senate Majority Leader Mitch McConnell grudgingly acknowledged that his side would not be able to block the deal but said it leaves Iran "with a threshold nuclear capability."
Israel also has railed against the deal, arguing that its conditions would keep Iran perilously close to developing nuclear weapons while enriching a government that has funded anti-U.S. and anti-Israel militants throughout the Middle East.
White House press secretary Josh Earnest called the growing support a validation of Obama's effort to "make sure that every member of the Senate understands exactly what's included in the agreement." The deal sets Iran back so that it is at least a year away from being able to produce enough nuclear material for a weapon, before the restrictions ease after a decade.
For all the geopolitical ramifications, the debate in the U.S. has often seemed more about domestic partisan politics over a resolution that, on its own, wouldn't be able to reverse a multi-country agreement already blessed by the United Nations. A vote of disapproval, however, could signal Congress' readiness to introduce new sanctions at the risk of causing Tehran — and other governments — to abandon the accord and blame the U.S. for the failure.
Republicans, defending their congressional majorities and aiming for the White House in next year's elections, have denounced the deal in apocalyptic terms. The bulk of Democrats have rushed to the president's defence.
Next week, Donald Trump and fellow presidential candidate Sen. Ted Cruz will rally outside the Capitol against the agreement, as lawmakers return from a five-week recess to begin debating it.
In the House, the disapproval resolution is certain to pass by a wide margin when it comes to a vote next week. But in a letter to fellow Democrats on Wednesday, House Minority Leader Nancy Pelosi said she has the votes to back up an Obama veto.
Supporters of the deal are seeking a bigger victory in the Senate. If they can assemble 41 votes in favour, they'd be able to block the disapproval resolution from passing at all, sparing Obama the embarrassment of having to veto it. They need seven of the remaining 10 undeclared Democrats to back the agreement, though several in this group could still come out in opposition.
Either way, Obama has succeeded in selling a package that prompted immediate and intense opposition from Republicans in the days after it was concluded on July 14 by Iran, the United States, Britain, France, Germany, Russia and China.
Millions were spent lobbying against the pact and polls registered significant public distrust. But none of the skepticism translated into enough Democratic opposition to threaten the deal, and only two Senate Democrats, Chuck Schumer of New York and Robert Menendez of New Jersey, have announced their opposition so far.