WASHINGTON — Prodded by President Donald Trump, a bitterly divided Senate voted, at last, Tuesday to move forward with the Republicans’ long-promised legislation to repeal and replace “ObamaCare.” There was high drama as Sen. John McCain returned to the Capitol for the first time after being diagnosed with brain cancer to cast a decisive “yes” vote.

The final tally was 51-50, with Vice President Mike Pence, exercising his constitutional prerogative, breaking the tie after two Republicans joined all 48 Democrats in voting “no.”

When the Senate voted Tuesday evening on the bill’s initial amendment, it underscored how hard it will be for the chamber’s divided Republicans to pass a sweeping replacement of Obama’s law.

By 57-43 — including nine GOP defectors — it blocked a wide-ranging proposal by Senate Majority Leader Mitch McConnell to erase and replace much of the statute. It included language by Sen. Ted Cruz, R-Texas, letting insurers sell cut-rate policies with skimpy coverage, plus an additional $100 billion to help states ease out-of-pocket costs for people losing Medicaid — a provision sought by Midwestern moderates including Rob Portman, R-Ohio.

At its most basic, the Republican legislation is aimed at undoing “ObamaCare’s” unpopular mandates for most people to carry insurance and businesses to offer it. The GOP would repeal “ObamaCare” taxes and unwind an expansion of the Medicaid program for the poor, the disabled and nursing home residents The result would be 20 million to 30 million people losing insurance over a decade, depending on the version of the bill.

The GOP legislation has polled abysmally, while “ObamaCare” itself has grown steadily more popular. Yet most Republicans argue that failing to deliver on their promises to pass repeal-and-replace legislation would be worse than passing an unpopular bill, because it would expose the GOP as unable to govern despite controlling majorities in the House, Senate and White House.

Tuesday’s vote amounted to a procedural hurdle for legislation whose final form is impossible to predict under the Senate’s byzantine amendment process, which will unfold over the next several days.

Indeed senators had no clear idea of what they would ultimately be voting on, and in an indication of the uncertainty ahead, McConnell, R-Ky., said the Senate will “let the voting take us where it will.” The expectation is that he will bring up a series of amendments.

Yet after seven years of empty promises, and weeks of hand-wringing and false starts on Capitol Hill, it was the Senate’s first concrete step toward delivering on innumerable pledges to undo former President Barack Obama’s law. It came after several near-death experiences for earlier versions of the legislation.