After seven years of emphatic campaign promises, Republicans senators on Wednesday failed to repeal "Obamacare", as the senate voted 55-45 to reject legislation undoing major portions of Barack Obama's healthcare law without replacing it.

Seven Republicans joined all Democrats in rejecting a measure by Republican Rand Paul of Kentucky that would have repealed most of former President Obama's healthcare law, with a two-year delay but no replacement.

Congress passed nearly identical legislation in 2015 and sent it to Obama, who unsurprisingly vetoed it.

Yet this time, with a president in the White House who's eager to sign the bill, the measure failed on the Senate floor.

The Congressional Budget Office has estimated that repealing "Obamacare" without replacing it would cost more than 30 million Americans their insurance coverage, and that was a key factor in driving away more Republican senators than Majority Leader Mitch McConnell could afford to lose in the closely divided Senate.

READ MORE: The Obamacare debate is not just about healthcare

The result frustrated other Republican senators, some of whom expressed disbelief that their colleagues would flip-flop on legislation they had voted for only two years ago and long-promised to voters. Of the current Republican senators, only moderate Susan Collins of Maine opposed the 2015 repeal bill.

"Make no mistake: Today's vote is a major disappointment to people who were promised full repeal," said Senator Ben Sasse of Nebraska. "We still have a long, long way to go - both in health policy and in honesty."

Yet the outcome was unsurprising in a Senate that's already shown that unity is elusive when it comes to dealing with Obamacare. The real-world implications of repeal have proven sobering to Republican senators answering to voters who've come to rely on expanded insurance coverage under the law.

What the party's senators will end up agreeing on instead is far from clear. Yet they plunged forward with debate towards their unknown goal, pressured by an impatient president. By the week's end, Republicans hope to reach agreement among themselves, and eventually with the House, on some kind of repeal and replacement for the Obama law they have reviled for so long.

A 'skinny repeal'

One possibility taking shape in talks among senators was a "skinny repeal" that would abolish just a few of the key elements of Obama's law including its mandates that everyone buy insurance and its taxes that all Republican senators can agree to oppose.

But in a sign of the general confusion, some said the tactic was aimed chiefly at moving the process forward into the purview of a committee of senate-house bargainers while others expressed the hope that the house would swallow a "skinny bill" whole, freeing Congress to move on to other issues.

Either way, after weeks spent on the issue including false starts and near-death experiences that have eaten up months of Trump's presidency, the realisation was dawning on senators that they may be unable to pass anything more complex for now than a lowest-common-denominator bill.

"At the end of the day, we've got to start somewhere. This is a start," said Republican Senator Thom Tillis.

The day's proceedings began with input from Trump, who's proven impatient and inconsistent throughout the healthcare debate and yet can claim some credit for resuscitating talks in the Senate after McConnell essentially declared them dead last week.

The president singled out Alaska Senator Lisa Murkowski, who'd voted the day before against opening long-awaited debate on the legislation, and also opposed a wide-ranging McConnell amendment on Tuesday that offered a replacement for Obamacare and went down to defeat.

"Senator @lisamurkowski of the Great State of Alaska really let the Republicans, and our country, down yesterday. Too bad!" Trump wrote.

"I don't really follow Twitter that much," Murkowski remarked to reporters later with a shrug.

Murkowski was also among the seven Republican senators who voted "no" Wednesday on the repeal-only bill. The others were Collins, Shelley Moore Capito of West Virginia, Dean Heller of Nevada, John McCain of Arizona, Rob Portman of Ohio and Lamar Alexander of Tennessee.

In a statement defending his vote, Portman wrote: "We need a rescue plan for Ohio families who are suffering under the status quo, not one that makes the healthcare system worse for Ohio families."