Republican US House Speaker John Boehner has described proposals to outlaw LGBT discrimination as an attack on small businesses, ahead of a Senate vote.
The Senate will vote later this evening on whether to enshrine the Employment Non-Discrimination Act (ENDA) into law.
There is growing optimism that Democratic Senate Majority Leader Harry Reid will be able to bypass a filibuster attempt.
Nevada Republican Dean Heller became the 60th member of the US Senate to back ENDA on Monday.
But Speaker John Boehner has warned Republicans against supporting the bill, effectively killing off its chances of progressing passed the Senate.
Even if ENDA clears the Senate, it faces immense hurdles in the House of Representatives where the GOP holds the balance of power.
“The Speaker believes this legislation will increase frivolous litigation and cost American jobs, especially small business jobs,” Mr Boehner’s spokesman Michael Steel said in a statement.
On Sunday, President Barack Obama called on Congress to pass the legislation. He said: “This bill has strong bipartisan support and the support of a vast majority of Americans. It ought to be the law of the land”.
President Obama does have the power to enforce ENDA via an executive order – but the White House remains adamant that it should be implemented with congressional approval.
White House Press Secretary Jay Carney reiterated the position on Friday.
Writing in the Wall Street Journal on Monday, Apple Chief Executive Tim Cook also called on Congress to pass ENDA. “Those who have suffered discrimination have paid the greatest price for this lack of legal protection,” he said.