WASHINGTON (REUTERS) - "We hope John McCain gets better very soon. Because we miss him," US President Donald Trump said at a White House economic event.

"He's a crusty voice in Washington, plus we need his vote."

Mr McCain, a critical vote in the United States Senate's healthcare overhaul, was described as upbeat and comfortable in Arizona on Monday (July 17), but was still days away from travelling to Washington after surgery to remove a blood clot from his head.

The Arizona senator underwent surgery last Saturday to remove a clot above his left eye discovered during a physical examination, forcing Senate majority leader Mitch McConnell to postpone a healthcare vote and throwing the timing of the debate into uncertainty.

Mr McCain's support is vital for the Bill, which needs the backing of at least 50 of the 52 Republicans in the 100-member Senate. Republicans Susan Collins and Rand Paul have already said they will not vote to open debate on the legislation - meaning one more "no" vote would kill it.

A friend who had talked to Mr McCain, 80, said "he sounds great" and that he was so eager to get back into the action in Washington that he was musing about the prospect of taking a cross-country road trip.

Changes in cabin pressure associated with air travel could harm the recovery for Mr McCain, who survived a bout of melanoma in 2000.

While his office has said the recovery could take a week, medical experts told The New York Times the recovery time for such procedures is often longer.

Even with Mr McCain, Republicans face plenty of obstacles to passage of the measure to replace the 2010 Affordable Care Act, popularly known as Obamacare.

Democrats are united in opposition to the Bill, and Ms Collins estimated on Sunday that "eight to 10" Republicans still have serious doubts about it.