The president was having lunch with some members of the Senate Finance Committee ahead of what was expected to be a series of contentious votes in the full Senate later in the week over proposed changes to the labyrinth USA tax code.
The Senate proposal permanently chops the corporate tax rate and temporarily reduces individual income taxes, while changing numerous deductions.
Trump tweeted just hours before a meeting with Senate Minority leader Chuck Schumer, D-N.Y., and top House Democrat Nancy Pelosi of California. Despite having threatened to oppose the bill, GOP senators Bob Corker of Tennessee and Ron Johnson of Wisconsin both voted it out of committee.
The Alexander-Murray bill has enough support in the Senate but has been blocked by GOP leadership. "And we know we will not be able to go forward until we get 50 people satisfied, and that's what we're working on".
The independent Congressional Budget Office concluded in a new report Sunday that the Senate version of the tax changes would give substantial cuts and benefits to those who earn more than $100,000 a year, while the country's poorest taxpayers would be worse off.
The CBO also estimated that the GOP tax bill could trigger automatic cuts worth $136 billion from mandatory spending in 2018, including $25 billion in Medicare cuts, if Congress doesn't find another way to offset its deficit increases.
Multiple other senators have expressed similar concerns to those of Corker and Johnson.
The legislation faces an open amendment process on the Senate floor that may spell substantial changes before a final vote that's expected Thursday.
It is not strictly necessary to pass the tax bill through the Senate Budget Committee, where Republicans are hoping to combine the tax bill with a provision allowing for oil and gas drilling in the Arctic National Wildlife Refuge.
Certainly, this White House wants to see it happen.
"Sometimes we assume we'll have a backbone later that we don't have now", he said, explaining his concern.
If the Senate approves its tax measure later this week, it would need to be reconciled with a version already approved by the House of Representatives before anything could be sent to the White House for Trump to sign into law.
Spokespeople for the Senate Finance Committee, which drafted the bill, did not immediately respond to a request for comment. The bill, he added, reflects "an incredible opportunity to promote the kind of lasting economic growth that can only come from enhancing free enterprise, increasing individual liberty, and expanding economic freedom". "I'm getting commitments we are going to get this fixed", he said.