US senator John McCain's absence from the Senate as he recovers from surgery for a blood clot has led the Republican leadership to postpone consideration of healthcare legislation already on the brink.
Senate majority leader Mitch McConnell said he was deferring action on the measure as Mr McCain recovers at his home in Arizona.
Surgeons in Phoenix removed a blood clot from above Mr McCain's left eye on on Friday.
The 80-year-old Senate veteran was advised by doctors to remain in Arizona until next week, his office said.
"While John is recovering, the Senate will continue our work on legislative items and nominations, and will defer consideration of the Better Care Act," Mr McConnell said in a statement.
A close vote had already been predicted for the Republican healthcare bill, with all Democrats and independents coming out against it and some Republicans opposed or undecided.
With the party holding a 52-48 majority, they can afford to lose only two Republicans. Vice president Mike Pence would break a tie for final passage.
Two Republicans, Rand Paul of Kentucky and Susan Collins of Maine, have already said they will vote against the measure.
A procedural vote expected in the coming days had been cast as a showdown over the measure designed to replace President Barack Obama's healthcare law, commonly called Obamacare.
Mr McConnell and other Republican leaders have been urging senators to at least vote in favour of opening debate, which would allow senators to offer amendments.
In recent days Republican leaders have expressed optimism that they were getting closer to a version that could pass the Senate.
In Phoenix, Mayo Clinic Hospital doctors said Mr McCain underwent a "minimally invasive" procedure to remove the nearly two-inch clot and that the surgery went "very well". Mr McCain was reported to be resting comfortably at his home in Arizona.
Pathology reports on the clot were expected in the next several days.
Mr McCain is a three-time survivor of melanoma. Records of his medical exams released in 2008 when he was the Republican candidate for president showed that he has had precancerous skin lesions removed and had an early stage squamous cell carcinoma, an easily cured skin cancer, removed.