WASHINGTON, April 7 (Reuters) - Democrats are aligning with Republicans to support a bill giving Congress the opportunity to approve or reject sanctions relief in an Iran nuclear deal, and are close to forming a veto-proof majority that U.S. President Barack Obama says could undermine the delicate final stage of negotiations.
The support for the legislation by lawmakers in Obama's party illustrates the depth of concern in Washington over the threat posed by Iran and the concern of many lawmakers that they are being shut out of the process to contain it.
In the wake of last week's announcement of an initial accord between Tehran and major world powers, senators are reaffirming their backing for the bipartisan bill and seeking ways to make the bill more palatable for the White House.
The Democrats, along with Republicans who control Congress, are pressing ahead despite White House claims that Obama alone has the power to negotiate and implement the evolving agreement that would see Iran curb its nuclear program in exchange for phasing out crippling sanctions. The deadline for a final deal is June 30.
The White House confirmed on Tuesday that Obama intended to veto the bill in its current form.
Even though Congress is in the midst of a spring break, Democratic senators have been toiling on the bill being crafted by Senate Foreign Relations Committee Chairman Bob Corker, a Republican, that could be approved by the panel next week.
"There's no way that Congress should allow the congressional sanctions regime to be negotiated away without saying a word," Democratic Senator Tim Kaine, who helped Corker write the legislation but who also supports the administration's nuclear negotiations with Iran, told Reuters.
Senator Chuck Schumer, one of the most influential Democrats and a co-sponsor of Corker's bill, has reaffirmed his support for a congressional role.
"I strongly believe Congress should have the right to disapprove any agreement and I support the Corker bill which would allow that to occur," he said on Monday.
Schumer, who is Jewish and represents New York with its more than 1.5 million Jews, is the third-ranking Senate Democrat and is expected to take over the party leadership in the chamber in 2017. Israeli Prime Minister Benjamin Netanyahu has railed against what he calls a "bad deal" and says Iran's nuclear ambitions are an existential threat to his country.
MOVES TO SOFTEN BILL
Under Corker's bill Congress would have 60 days to review the agreement, during which sanctions relief would be suspended and lawmakers could vote on whether to approve or reject sanctions measures.
Corker has already agreed to change the wording so that a lack of action by Congress would count as approving the deal, and that Congress could only weigh in on relief of congressional sanctions, not the entire deal. Kaine said those changes were made at his request.
In coming days, the White House and allies in Congress could seek ways to soften Corker's legislation further with steps such as simply requiring regular reports to lawmakers on progress in implementing the deal, coupled with an expedited process for reinstating sanctions if Iran violates its terms.
Sanctions relief has been one of the key sticking points in the marathon talks that could yet sabotage a final deal. The White House has said sanctions would be phased out but Iran's negotiators have interpreted the accord differently, saying they would be lifted immediately.
The Obama administration argues that the bill would interfere with the talks and deter Iran from signing a deal that it sees as potentially ending decades of tense relations with Iran and possibly fostering broader Middle East peace.
But Obama took a more conciliatory line in an interview with the New York Times at the weekend, saying he hoped Congress could "express itself" without encroaching on "traditional presidential prerogatives."
With most or all of the 54 Republican senators expected to back the bill and nine Democrat co-sponsors, the 60 votes needed to take it through the Senate seem assured. It would likely get a sympathetic reception in the Republican-majority House, and then reach Obama's desk.
Congress could override a presidential veto with a two-thirds vote in both the Senate and House of Representatives, in what would be an embarrassing setback for Obama. In the Senate, that would require 67 votes.
In addition to the nine Democrat co-sponsors, one independent has co-sponsored the bill, another Democrat has put out a statement supporting it, and several others have signaled they are open to backing it.
Obama faces a tough battle because at stake is congressional oversight of a potentially landmark deal with a foreign country. Nonetheless, Democrats are warning that they could drop their support if Republicans let partisan politics sneak into the Corker bill.
"If I become convinced...that the bill as amended, given the debate, is really nothing more than a partisan vehicle for killing the prospects for a deal, I won't support that," said Democratic Senator Chris Coons, a Foreign Relations Committee member who has signaled potential support of the Corker legislation.
So far, some leading Democrats see Corker as an honest broker.
Senator Benjamin Cardin, who recently became the senior Foreign Relations Democrat said the revised Corker bill was an "orderly way" for Congress to review the agreement, giving it the option to refrain from action and thus let the deal stand.
Of the framework deal, Cardin said in an interview with Reuters: "It is too early to predict whether this agreement is the best deal we can get" in keeping nuclear weapons out of Iranian hands. (Additional reporting by Patricia Zengerle, writing by Richard Cowan; editing by Stuart Grudgings.)