(Adds U.S. Capitol Police on number of protesters arrested, comment from Cornyn on timing of vote)
WASHINGTON, July 10 (Reuters) - President Donald Trump on Monday prodded the Republican-led U.S. Congress to pass major healthcare legislation but huge obstacles remained, with a senior lawmaker saying the Senate was unlikely to take up the stalled bill until next week.
The House of Representatives approved its healthcare bill in May but the Senate's version appeared to be in growing trouble as lawmakers returned to Washington from a week-long recess.
"I cannot imagine that Congress would dare to leave Washington without a beautiful new HealthCare bill fully approved and ready to go!" Trump wrote on Twitter, referring to the seven-year Republican quest to dismantle Democratic former President Barack Obama's signature legislative achievement.
Lawmakers are set to take another recess from the end of July until Sept. 5.
Repealing and replacing the Affordable Care Act, dubbed Obamacare, was a central campaign pledge for the Republican president. But Senate Republican leaders have faced a revolt within their ranks, with moderate senators uneasy about the millions of Americans forecast to lose their medical insurance under the legislation and hard-line conservatives saying it leaves too much of Obamacare intact.
They were struggling to find a compromise that could attract the 50 votes needed for passage in a chamber Republicans control by a 52-48 margin, with Vice President Mike Pence casting a potential tie-breaking vote in the face of unified Democratic opposition.
No. 2 Senate Republican John Cornyn said Republicans could release an updated draft of their bill by the end of the week and told Fox News that senators could vote as early as Tuesday or Wednesday of next week.
"We're going to continue to talk and listen and exchange ideas on how we can continue to make improvements," Cornyn said on the Senate floor.
Also speaking on the Senate floor, Majority Leader Mitch McConnell gave no timetable for the bill. McConnell signaled his determination to keep working and said mere legislative "band-aids" would not suffice.
Senate Democratic leader Chuck Schumer said he had written to McConnell urging a bipartisan effort to stabilize the health insurance market, noting that McConnell had been quoted recently as saying Congress would need to shore up that market if lawmakers fail to repeal Obamacare.
The U.S. Centers for Medicare and Medicaid Services issued data on Monday showing a 38 percent decrease in applications by insurers to sell health plans in the Obamacare individual market in 2018 compared to this year. The agency said insurers continue to flee the exchanges, the online marketplace for health insurance set up under Obamacare.
MORE AMERICANS UNINSURED
With uncertainty hanging over the healthcare system, the percentage of U.S. adults without health insurance grew in the April-May-June period to 11.7 percent, up from 11.3 percent in the first quarter of 2017, according to Gallup-Sharecare Well-Being Index figures released on Monday. That translates into nearly 2 million more Americans who have become uninsured.
Scores of protesters voiced opposition to the legislation outside the Republican National Committee headquarters and at the offices of some Republican lawmakers including House of Representatives Speaker Paul Ryan, chanting slogans including "Trumpcare kills" and "Healthcare is a human right."
U.S. Capitol Police said in a statement 80 people were arrested at 13 locations in House and Senate office buildings after they refused "to cease and desist with their unlawful demonstration activities."
Republicans criticize Obamacare as a costly government intrusion into the healthcare system. Democrats call the Republican legislation a giveaway to the rich that would hurt millions of the most vulnerable Americans.
The Senate legislation would phase out the Obamacare expansion of the Medicaid health insurance program for the poor and disabled, sharply cut federal Medicaid spending beginning in 2025, repeal most of Obamacare's taxes, end a penalty on Americans who do not obtain insurance and overhaul Obamacare's subsidies to help people buy insurance with tax credits.
Leerink Partners analyst Ana Gupte said investors remained in a "wait-and-see" mode regarding the Senate legislation.
(Additional reporting by Susan Heavey, Eric Beech and Doina Chiacu; Writing by Will Dunham; Editing by Tom Brown)