Trump plea for unity on showdown healthcare vote
US President Donald Trump has issued a challenge to Republican senators to rally behind their healthcare bill ahead of a crucial vote.
"Now is the time for action," he said in a rare healthcare speech on Monday.
A procedural vote is taking place as Republicans push forward in their attempt to repeal former 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 before the vote: "I don't have a clue what we're going to be voting on."
What is the vote on Tuesday about?
Senators are voting on a procedural motion to start debate on a healthcare bill.
Republicans 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 procedure 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 Rand Paul, who opposes the repeal-and-replace bill, tweeted on Tuesday that he would vote in favour of the motion after receiving assurances from Mr McConnell that they planned to take up a clean repeal vote.
But he also warned that this vote could fail, as it requires 60 votes, and suggested there may even be a third vote on "whatever version of clean repeal we can pass".
He is referring to a so-called "skinny bill", a far narrower measure that would scale back some of the more controversial elements in an effort to get a wider consensus.
Republican senators are due to meet to discuss their options over lunch, before the vote takes place.
How likely is it the bill will pass?
Whichever way it goes, it will be tight.
Republicans need a majority for the motion to go ahead and have only 52 seats in the 100-seat Senate.
A breakthrough came for Republicans when Senators Rand Paul, Shelley Moore Capito and Dean Heller, all opponents of the Senate's recent Obamacare repeal plans, said they were in favour of debating a healthcare bill.
Crucially, Senator John McCain, who was recently diagnosed with an aggressive brain tumour, has returned to Congress to cast his vote.
Mr McConnell is hoping his presence will help push the vote through, as the Arizona senator has previously voiced support for debating the healthcare bill.
The Republican senator of Maine, Susan Collins, had already stated she would vote against the bill, and several others have expressed their reservations.
What did President Trump say?
The president increased the pressure on his party by warning them they had a duty to adhere to seven years of promises.
In a White House speech laced with frustration, President Trump said: "To every member of the Senate I say this: The American people have waited long enough.
"There's been enough talk, and no action. Now is the time for action."
He lambasted his predecessor's overhaul of healthcare, known as Obamacare, and stood alongside people he said were "victims" of the 2010 law.
Obamacare extended healthcare insurance to about 20 million people but Republicans hate the way it introduced government-run marketplaces, where premiums have risen sharply for some people.
Mr Trump tweeted again on Tuesday morning, saying it was a "big day for healthcare" and that "we will soon see whether or not Republicans are willing to step up to the plate!"
He added: "Repeal or Repeal & Replace! I have pen in hand."
What have Republicans proposed?
The party's proposed alternative includes steep cuts to Medicaid, a healthcare programme for the poor and disabled.
And it removes Obamacare's individual mandate requiring all Americans to have health insurance or pay a tax penalty.
The non-partisan Congressional Budgetary Office (CBO) found the bill would strip 22 million Americans of health insurance over the next decade.
But if the bill gets to the floor of the Senate, amendments could be added to change the various provisions.
And what's this stuff about a duel?
A Republican politician has blamed "female senators" for the spluttering efforts by his party to pass a healthcare bill.
Blake Farenthold, a congressman from Texas, told a local radio station if a man from his state was responsible, he would challenge him to a duel.
"Some of the people that are opposed to this, they're some female senators from the North East."
Maine senator Susan Collins was one of several female Republicans who said they could not back the Senate's last healthcare plan, a repeal with two-year delay, which hit the buffers last week.
Ms Collins, a moderate Republican, has been strongly opposed to the Republican bill from the start over the cuts to Medicaid.
Republican Senator Lisa Murkowski of Alaska also spoke out against the bill, expressing concerns over plans to weaken protections for people with pre-existing conditions and affordability.
There was an outcry when Republican Senate leader Mitch McConnell drafted the first healthcare bill in private among a group of 13 that did not include any women.