US Republican senator Rand Paul appeared today to back the Trump administration's sweeping tax cut plan.

Rand Paul said he was "all in" for massive tax cuts even as the Senate passed a key budget measure without his support one day earlier. 

The Republican-controlled Senate approved the budget resolution for the 2018 fiscal year yesterday with Paul casting the lone Republican vote against it. 

That approval paves the way for their tax-cut proposal that would add up to $1.5 trillion to the federal deficit over the next decade to pay for the cuts. 

US President Donald Trump today signalled optimism for passage of the cuts, saying Paul would back the proposed tax measure when it comes up for a vote.

Paul had said he would not vote for the budget measure unless it in kept in line with previously enacted federal budget spending caps. 

Republicans are still hammering out their tax legislation after releasing an initial outline. The administration has said it would deliver up to $6 trillion in tax cuts to businesses and individuals. 

Republicans also control the US House of Representatives and the White House.

They are under pressure to succeed on tax reform after failing to make good on another key agenda item, their years-long pledge to scrap Obamacare, the signature healthcare law of Democratic former President Barack Obama. 

Trump is also seeking his first major legislative victory since taking office in January amid a tumultuous tenure overshadowed by ongoing investigations into Russia's alleged interference in the 2016 election, as well as a series of the president's comments that have sparked controversy. 

Democrats remained united in their opposition to the budget bill and are unlikely to support the Republicans' tax plan.

They argue that it would benefit the wealthy, raise taxes on some middle-class Americans and widen the federal deficit.