(Adds Biden's conversation with Pelosi and Schumer)

By Nandita Bose

Sept 16 (Reuters) - U.S. President Joe Biden on Thursday expressed confidence that Congress will pass both a bill funding infrastructure investments and a supplementary spending bill as Democrats seek to infuse trillions of dollars into the U.S. economy.

Democrats in Congress are writing a $3.5 trillion spending bill that funds child care, community college and other social programs with an increase in taxes on companies and the very wealthy. The party seeks to pass the massive package as a companion to a $1 trillion infrastructure bill that has bipartisan support.

"I know we still have a long way to go, but I’m confident that Congress will deliver to my desk both the bipartisan infrastructure plan and the Build Back Better Plan I proposed," he said, using the spending bill's name.

Biden later had a "positive discussion" with House of Representatives Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer about progress in advancing the bills, the White House said.

They also discussed plans to pass a continuing resolution to fund government operations, the White House said. The resolution must be passed by the end of September to keep government operations funded.

While the infrastructure plan has support on both sides of the aisle, Democrats in the Senate, where they narrowly hold control, face a tough battle to finalize the plan that would expand the social safety net.

As Republicans decline to support or negotiate on the $3.5 trillion package, Senate Democrats are taking a route known as budget reconciliation to pass the bill with a simple majority vote.

The party must convince moderate members in its own ranks, like Senators Joe Manchin of West Virginia and Kyrsten Sinema of Arizona, to back the bill. Both have raised questions about the size and scope of the program. (Reporting by Nandita Bose; Writing by Tyler Clifford; Editing by Cynthia Osterman and Stephen Coates)