WASHINGTON, Oct 18 (Reuters) - Negotiators of a U.S. bill to invest up to $3.5 trillion to expand social programs and attack climate change gave hints of progress on Monday, but some Democrats were resigned to the increasing likelihood that a proposal to reduce carbon emissions will be weakened or scrapped. "Over the last weekend I held many productive conversations" with lawmakers and the White House, Senate Majority Leader Chuck Schumer, a Democrat, said. "We still have work to do," he added, without providing details in a Senate floor speech. The legislation is a pillar of President Joe Biden"s domestic agenda. He was set to speak on Monday with Democratic Senator Joe Manchin, whose support for the wide-ranging proposal is key to its passage. Manchin, a moderate from West Virginia, has been an outspoken critic of the bill, saying it spends too much taxpayer money and contains climate change provisions that would hurt his state"s coal mining industry. Democratic Senator Sheldon Whitehouse, a leading voice for reducing emissions blamed for the wildfires, floods and other natural disasters now confronting the warming planet, was asked by reporters whether the Clean Energy Payment Program (CEPP) included in Biden"s $3.5 trillion initiative was in jeopardy. "It remains to be seen but I think it"s worth planning for that," Whitehouse said. Several Democratic senators were already touting other steps contained in the bill aimed at lowering greenhouse gas emissions in a sign that the CEPP, which would reward utilities that add more clean energy capacity like solar and wind power and fine those that do not, was in trouble. "I think it makes the methane pollution fee and the carbon pollution fee all the more essential," Whitehouse added. Moderate Democratic Senator Mark Warner urged prompt House of Representatives passage of a separate, $1 trillion infrastructure investment bill already approved by the Senate, while awaiting a deal on the larger bill. "People want us to put points on the board," Warner told reporters. He gave as examples the need to pass the infrastructure measure or a bill making major high-tech investments so that the United States can compete more effectively with China. The Virginia Democrat added that doing so would boost the candidacy of Democrat Terry McAuliffe"s gubernatorial bid in that state in next month"s election. With virtually no chance of winning over any of the Senate"s 50 Republicans for the larger bill, Democrats are employing a special "budget reconciliation" strategy that would allow them to waive a 60-vote threshold required to advance most bills in the 100-member U.S. Senate. All 50 Democrats would have to vote "yes" on the bill, with Democratic Vice President Kamala Harris on hand to break any 50-50 ties. Another Democratic moderate, Senator Kyrsten Sinema, also has voiced opposition to the bill as currently written. House of Representatives Speaker Nancy Pelosi is aiming for passage of the social investment bill and the $1 trillion infrastructure bill by the end of this month. Reporting by Richard Cowan and David Morgan; Editing by Matthew Lewis and Peter Cooney Our Standards: The Thomson Reuters Trust Principles.