Senate Republican leaders are trying to send a revised version of their healthcare bill to the Congressional Budget Office (CBO) by Friday, so its impact can be assessed.

Majority Leader Mitch McConnell had originally been aiming to hold a vote on the measure to repeal and replace Obamacare this week. But he was forced to change his schedule in the face of growing opposition from members of his own party.

Mr McConnell is now under pressure to get the Republicans’ healthcare overhaul passed before Congress's August recess – as Senate Minority Leader Chuck Schumer works to keep the Democrats mobilised to defeat the legislation.

The majority leader is apparently trying to move quickly so that the nonpartisan CBO has time to measure the revised bill’s impact on coverage levels and federal spending by the time senators return to Washington DC in mid-July after the 4 July recess.

Donald Trump told reporters on Wednesday that “healthcare is working along very well...we're gonna have a big surprise. We have a great healthcare package.”

When asked what that meant, Mr Trump responded: “We're going to have a great, great surprise.”

Mr Trump summoned all 52 Republican senators to the White House on Tuesday afternoon to discuss how to proceed after the vote postponement, with the vast majority attending.

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During his opening remarks, the President declared that Obamacare “is melting down”, adding that Senate Republicans and the White House are "getting very close" to reaching agreement on a path toward getting the votes needed to pass the latest healthcare bill.

“This will be great if we get it done,” Mr Trump said. But appearing to recognise the opposition the bill faces, he added: “And if we don't get it done, it's just going to be something that we're not going to like. And that's okay, and I understand that very well.”

It is still unclear which parts of the bill, officially titled the Better Care Reconciliation Act, are being revised.

Republican legislators' overlapping concerns and competing interests have presented Mr McConnell with a balancing act. Moderate senators worry that millions of people would lose their insurance, while conservatives assert that the bill does not do enough to erase Democratic former President Barack Obama's signature domestic legislation.

Acknowledging demands from fellow Republicans for increased input into retooling the legislation, Mr McConnell said on the Senate floor: “Senators will have more opportunities to offer their thoughts as we work toward an agreement.”

With Democrats unified against it and Republicans controlling the Senate by a slim 52-48 margin, Mr McConnell can afford to lose only two Republican senators. At least 10 Republican senators – including moderates, hard-line conservatives and others – have expressed opposition to the current bill, although some have said they want to vote for it if certain changes are made.

Mr McConnell, with his reputation as a strategist on the line, met with a procession of Republican senators in his office, including some who have criticised the bill. John Cornyn, the No 2 Senate Republican, said the party leadership would be talking to every Republican senator who had concerns about the bill or was undecided.

Mr Cornyn said it would be “optimal” to have changes to the legislation worked out by Friday so a new version could be analysed by the CBO. The CBO forecast on Monday that the existing bill would lead to some 22 million people losing medical insurance by 2026 than under Obamacare – all while cutting the federal deficit by $321 billion.

The prospect of so many people losing insurance is unpalatable to moderates such as Senator Susan Collins, a key opponent of the draft legislation. She said it would be “very difficult” to reach agreement on the bill by Friday.

Mr Trump rebuffed a proposal by Senate Democratic leader Chuck Schumer for the Republican President to call all 100 senators to the Blair House, across the street from the White House, to craft a bipartisan healthcare bill to fix, not gut, Obamacare. Mr Trump said he did not think Mr Schumer was serious.

The bill has drawn protesters, including cancer survivors and people with disabilities, to the offices of several Senators, trying to convince them to reject the Senate bill. Capitol Police said they had arrested 40 people on charges of crowding and obstructing the offices.