The Senate deadlocked 50-50 on moving forward with the healthcare debate, forcing Vice President Mike Pence to cast the tie-breaking vote.

Senator John McCain, who was diagnosed this month with brain cancer and has been recovering from surgery at home in Arizona, made a dramatic return to the U.S. Capitol to cast a crucial vote in favor of proceeding.

The outcome was a huge relief for President Donald Trump, who had pushed his fellow Republicans hard in recent days to live up to the party's campaign promises to repeal the 2010 Affordable Care Act, commonly known as Obamacare. Minutes after the vote, Trump called it "a big step."

But the narrow victory on a simple procedural matter raised questions about whether Republicans can muster the votes necessary to pass any of the various approaches to repeal.

Moderates are worried repeal will cost millions of low-income Americans their insurance and conservatives are angry the proposed bills do not go far enough to gut Obamacare, which they consider government overreach.

In a first vote of the many likely to come this week, the plan to repeal and replace Obamacare that Senate Republicans have been working on for months failed to get the 60 votes needed for approval on Tuesday night. The vote was 43 in favor and 57 against.

Nine Republicans, ranging from moderates such as Susan Collins of Maine to conservatives such as Rand Paul of Kentucky, voted against the bill, which would have made deep cuts to Medicaid, the health insurance program for the poor, and reduced Obamacare subsidies to lower-income people to help them defray the cost of health insurance.

Earlier McCain, 80, received an ovation from his fellow senators when he entered the chamber to cast a vote to open debate. After that vote, he decried growing partisanship in the Senate and urged members to learn how "to trust each other again."

Collins and Senator Lisa Murkowski were the only Republicans to oppose the measure to open debate, and with Republicans controlling the Senate by a 52-48 majority, those were the only votes the party leadership could afford to lose. Democrats were united in opposition to the motion to proceed.