"Now is the time for action," President Trump said in a rare healthcare speech.
A procedural vote is due on Tuesday as Republicans push forward in their attempt to repeal President Barack Obama's signature law. But among Republicans, there is confusion about the bill and it is unclear whether it will pass.
Senator Ron Johnson told reporters: "I don't have a clue what we're going to be voting on."
What is the vote on Tuesday about?
Senators will vote on a procedural motion to start debate on a health care bill.
They have a majority in the Senate but there are deep divisions in their party on the issue.
"Many of us have waited literally years for this moment to arrive and, at long last, it finally has," said Republican Senate leader Mitch McConnell on the Senate floor.
But several senators have expressed unhappiness over the process because it is not clear what they will vote on if the procedural vote passes.
There appear to be two choices - either a repeal-and-replace bill that has already struggled to win support across the party or a bill that enacts repeal with a two-year delay, in the hope of finding agreement before that time elapses.
Senator John McCain, recently diagnosed with an aggressive brain tumor, will return to Congress to cast his vote.