Thirty-two million Americans would lose health coverage under a Republican plan to repeal Obamacare, the Congressional Budget Office (CBO) has forecast.
The non-partisan office's analysis found the cost of a medical insurance policy would increase 25% next year and double by 2026.
The repeal bill would also cut the federal deficit by $473bn (£363bn), predicted the CBO.
The Republican-controlled Senate has twice failed to pass a healthcare bill.
Its members plan to vote next week on a plan to repeal President Barack Obama's 2010 health law with a two-year delay.
But the CBO estimates the number of uninsured would rise by 17 million next year alone if the Affordable Care Act, known as Obamacare, were to be overturned.
President Donald Trump earlier called on his party to postpone their summer holiday until they have repealed Obamacare and replaced it with the Republican plan.
Trump told 49 Republican senators at the White House: "We should hammer this out and get it done."
In the last two days he has switched position several times, urging the repeal and replace of Obamacare, just repealing it, allowing it to fail, before reverting to repeal and replace on Wednesday.
Trump said: "For seven years you promised the American people that you would repeal Obamacare.
"People are hurting. Inaction is not an option. And frankly I don't think we should leave town unless we have a health insurance plan."
Trump warned a senator who was seated next to him that he could lose his job if he did not toe the party line.
A ripple of uncomfortable laughter was heard in the room as the president said of Nevada's Dean Heller: "And he wants to remain a senator, doesn't he? OK."
Heller, who was one of the earliest senators to oppose the first version of the Republican health bill, is up for re-election next year.
Senate Republican leader Mitch McConnell has scheduled a vote early next week on a straight-up repeal of Obamacare.
However, it looks likely to fail after the defections on Tuesday of at least three of the party's senators.