Republicans in the US Senate scrambled to salvage their tax plan Thursday after it hit unexpected roadblocks, leaving President Donald Trump still waiting for a major political victory he hopes to secure before year's end.

Trump had hoped his party would pass the controversial bill as early as Thursday night, handing the White House and Republicans their most significant legislative achievement of his young presidency.

But after 11th-hour negotiations over how to offset the cost of dramatic tax cuts for corporations and more modest cuts for individuals, Senate Majority Leader Mitch McConnell postponed votes until Friday midday.

A tense standoff on the Senate floor threw the bill's fate into question, after a new projection showed the measure would add $1 trillion to the federal deficit.