WASHINGTON, March 5 (Reuters) - A sharply divided U.S. Senate will begin a contentious debate on Friday on a $1.9 trillion coronavirus aid bill that is President Joe Biden’s first major legislative initiative, with Democrats pressing ahead without any Republican support. The Senate voted on Thursday to take up the bill in a party-line 51-50 vote, with Democratic Vice President Kamala Harris breaking the tie. But Republicans delayed the start of the debate by forcing the hours-long reading of the full text of the 628-page measure. The Senate is expected to debate the bill three hours on Friday before considering a multitude of amendments, which could require a marathon voting session, before taking a vote on final passage in a process that could extend into the weekend. Republicans are expected to use procedural maneuvers to slow the process. If the Senate approves the bill, it will have to be sent back to the Democratic-controlled House of Representatives for final passage. Democrats hope Biden can sign the bill into law before March 14, when some of the current benefits run out. With no votes to spare, Senate Democrats have tweaked the measure to ensure all 50 of their members would support it. Those changes would steer more aid to smaller U.S. states and add money for infrastructure projects, among other adjustments. But efforts by some senators to alter temporary federal unemployment benefits failed. The Senate bill keeps the House plan for $400 per-week payments through Aug. 29. It was unclear whether any senators would try to change that figure, possibly to $300, during the amendment process in coming days. “The time is now to move forward with big, bold, strong relief for the American people,” said Democratic Senate Majority Leader Chuck Schumer, who has promised to keep the Senate at work until the bill is passed. Republicans, who broadly backed COVID-19 relief spending early in the pandemic, have criticized the bill’s price tag. The relief legislation includes funding for vaccines and medical supplies, extends jobless assistance and provides a new round of emergency financial aid to households, small businesses and state and local governments. Opinion polls indicate broad public support. Senate Democrats on Wednesday tightened criteria for stimulus checks so fewer high-income households would qualify. The compromise means that 9 million fewer households would receive a stimulus payment than in the last tranche of payouts in 2020. It also lowers the cost of the legislation by $12 billion, according to Senate Democrats. On Thursday, they said they had increased minimum payments to states with smaller rural populations to match the $1.25 billion minimum contained in last year’s CARES Act. The bill passed by the House set the floor at $500 million. The pandemic has killed nearly 520,000 Americans and thrown millions out of work, although infection rates have eased in recent weeks as more people are vaccinated. The Senate’s No. 2 Republican, John Thune, said larger states like California, New York and Illinois still got the lion’s share in the “big, bloated, wasteful bill.” “You’ve got taxpayers in places like South Dakota and North Carolina and Georgia and other places around the country that essentially are writing checks to states which really aren’t needed,” Thune told PBS. Democrats also included $10 billion for infrastructure, $8.5 billion for health providers and expanded healthcare subsidies for those who lose their jobs. In the Senate, bills usually require the support of 60 senators. But the coronavirus relief bill is being advanced under a legislative maneuver known as reconciliation that allows passage with a simple majority vote.