(Adds no immediate comment from senator's office)
WASHINGTON, Oct 20 (Reuters) - U.S. President Donald Trump on Friday signaled optimism for the passage of Republicans' sweeping tax cut plan, saying a key senator who rejected the party's budget blueprint a day earlier would back the proposed tax measure when it comes up for a vote.
"The Budget passed late last night, 51 to 49. We got ZERO Democrat votes with only Rand Paul (he will vote for Tax Cuts) voting against," Trump wrote on Twitter. "This now allows for the passage of large scale Tax Cuts (and Reform), which will be the biggest in the history of our country!"
Representatives for Paul could not be immediately reached for comment on whether he would support the tax legislation, which Republicans are still hammering out after releasing an initial outline.
On Thursday, the Republican-controlled Senate approved the budget resolution for the 2018 fiscal year, with Paul casting the lone Republican vote against it.
That approval paves the way for their tax-cut package, which would add up to $1.5 trillion to the federal deficit over the next decade in order to pay for the proposed cuts.
Republicans, who also control the U.S. House of Representatives and the White Hours, are under intense 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 looking to score 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 Republicans' tax plan, which they argue would benefit the wealthy, raise taxes on some middle-class Americans and widen the federal deficit.
Trump made clear his preference for a bipartisan tax bill on Wednesday in a meeting with members of the Senate Finance Committee from both parties, Republican Senator John Cornyn told reporters.
The Trump administration has said its tax plan would deliver up to $6 trillion in tax cuts to businesses and individuals.
The Senate's budget must be reconciled with a markedly different version passed by the House, a process lawmakers have said could take up to two weeks. (Reporting by Susan Heavey; Editing by Mark Heinrich and Bernadette Baum)