Republican senators voted Tuesday to open debate on revising U.S. health care law, advancing a signature campaign pledge following a tense and dramatic week on Capitol Hill. But the party voted to do so only with the barest of margins, an illustration of the challenges ahead as leaders look to unite a divided and unruly caucus.

Johnny Isakson and David Perdue, Georgia’s two GOP senators, provided two much-needed votes for Senate Majority Leader Mitch McConnell, who had worked furiously over the last several days to secure the required GOP votes to kick off debate on repealing Obamacare. The final vote of the full Senate was 51-50 after Vice President Mike Pence broke the tie.

Tuesday's action kicks off days of debate on the Senate floor, but the road ahead for the GOP's years-long effort to nullify the 2010 health care law is murky. There are still deep divides among Republicans about which versions of repeal or replace are politically acceptable, particularly regarding Medicaid. And it's unclear if 50 GOP senators can agree on any single proposal.

McConnell has discussed voting on several different proposals in the days ahead, including the Senate's Obamacare replacement bill. The chamber on Tuesday evening rejected a version of that plan that would also allow the sale of cheap, bare-bones insurance plans. It's on track to vote today on a proposal would erase much of the Affordable Care Act and give Congress two years to settle on a replacement.

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