WASHINGTON (Reuters) - Republican senators returned to Washington on Monday following a 10-day holiday recess still at odds with one another over legislation President Donald Trump wants passed to repeal major portions of Obamacare. With only three weeks left before a summer recess scheduled to stretch until Sept. 5, Senate Majority Leader Mitch McConnell appeared determined to keep trying to find agreement on a partisan, all-Republican bill. If he cannot, he will be faced with giving up on a seven-year Republican promise to repeal the 2010 Affordable Care Act, popularly known as Obamacare - and possibly turning to Democrats for help in fixing problems with U.S. health insurance markets. The Republican legislation would phase out the Obamacare expansion of Medicaid health insurance 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. Democrats call the Republican legislation a giveaway to the rich that would hurt millions of the most vulnerable Americans. Since the start of 2017, when they took control of the White House, Republicans who also control Congress have been struggling over how to replace former Democratic President Barack Obama’s signature legislation - which they see as  government intrusion - with less comprehensive federal healthcare provisions. Conservative Republican Senator Ted Cruz favors offering a choice of cheaper healthcare plans that would not have to meet minimum standards established by Obamacare, such as coverage for maternity care and prescription drugs. Moderate Republican Senator Susan Collins dismissed that approach on Monday, telling reporters: “I believe that it would cause further destabilization of the individual (insurance) market, it would erode protections for people with pre-existing conditions” and cause premiums to rise. Earlier on Monday, Trump used Twitter to prod Republican senators to pass a bill, and senior lawmakers, including the Senate’s No. 2 Republican, John Cornyn, said legislation could advance to votes next week in the deeply divided chamber.  “I cannot imagine that Congress would dare to leave Washington without a beautiful new HealthCare bill fully approved and ready to go!” tweeted Trump, who made repealing and replacing Obamacare a central pledge of his 2016 campaign. The Republican-controlled House of Representatives approved its healthcare bill in May, knowing the Senate would amend it. Republican Senator John Hoeven of North Dakota told reporters he was worried about how rural hospitals would be treated in the Senate bill, which is being assembled in secret. He said lawmakers were awaiting an assessment of a retooled Senate bill from the non-partisan Congressional Budget Office before making decisions. Senior Republican Orrin Hatch said he did not yet know whether the minimum 50 Republican senators would come together on a bill, with Vice President Mike Pence casting the potential tiebreaking vote. Democratic leader Chuck Schumer wrote 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 with this year. Scores of protesters voiced opposition to the legislation outside Republican National Committee headquarters and at the offices of some Republican lawmakers, chanting slogans including “Trumpcare kills” and “Healthcare is a human right.”  U.S. Capitol Police said 80 people were arrested. (For a graphic on who's covered under Medicaid, click bit.ly/2u3O2Mu) 