West Virginia Sen. Joe Manchin III, one of the chamber's most conservative Democrats, said he backed the filibuster because he shared Ms. Warren's opposition to loosening banking rules imposed after the 2008 financial crisis.
"I have a lot of the same beliefs. We just want a level playing field. You know, the table has been tilted and gives advantages to people who don't need advantages," said Mr. Manchin. "I'm sure on other issues she's much farther left than I would ever be, but on that and on fiscal and financial issues [we agree]."