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!"

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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 proposed cuts.

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 increase the federal deficit.

The Senate's budget must be reconciled with a markedly different version passed by the U.S. House of Representatives, also controlled by Republicans, a process lawmakers have said could take up to two weeks.