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 US 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.

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 programme in exchange for phasing out crippling sanctions. The deadline for a final deal is June 30.

The White House has confirmed that Obama intended to veto the Bill in its current form. 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, said.

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.

The Obama administration argues that the Bill would interfere with the talks and deter Iran from signing a deal that may end decades of tense relations with Iran.

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.