The US Senate minority leader Chuck Schumer has described as "a sham" a plan to lift sanctions affecting the Aughinish Alumina plant in Co Limerick.
Last month, the US Treasury said it would end the sanctions that had been imposed on Rusal, the Russian parent company of Aughinish Alumina.
As part of the deal, Rusal agreed to reduce the ownership stake held by oligarch Oleg Deripaska.
Senator Schumer, the most senior Democrat in the US Senate, said at the weekend that he would force a vote to block the lifting of sanctions and today he urged Republicans to at least debate a resolution of disapproval.
"The case against the Treasury Department's proposal is strong," said Senator Schumer. "This is a sham. It's a sham to do a favor not only to Deripaska, but to Putin.
"If the Republicans are scared to even debate something as rudimentary as this, shame on them," he added.
A vote in the US Senate could take place this evening but Democrats do not have a majority and would require the support of some Republicans in order to block the lifting of sanctions.
Earlier, the second-ranking Democrat in the House of Representatives Steny Hoyer said he would call Treasury Secretary Steven Mnuchin today and ask him to delay lifting the sanctions until 28 February to give Congress more time to review the plan.
Aughinish Alumina employs 460 staff, 200 contractors and indirectly supports hundreds of additional jobs but there have been fears over the future of the plant because of the sanctions.
The Irish Government has been lobbying US officials to highlight the impact on the company.
Earlier this month, a number of European ambassadors, including Ireland's representative in Washington Dan Mulhall, sent a letter to Congress urging members to support the lifting of sanctions.
In recent days however Democrats have voiced their opposition to the move, accusing the Trump administration of being soft on Russia.
Last week, the US Treasury Secretary Steven Mnuchin briefed members of the House of Representatives on the matter.
After Democrats expressed concerns, Secretary Mnuchin suggested he was open to extending the 17 January deadline for lifting the sanctions to give members of Congress more time to review the plan.