By Sarah N. Lynch WASHINGTON (Reuters) - Centrist Democratic Senator Joe Manchin, a pivotal vote in the U.S. Senate, on Sunday advocated making the procedural maneuver called the filibuster more "painful" to do, with Democrats concerned about Republicans obstructing President Joe Biden's legislative agenda. Some Democrats have advocated eliminating the filibuster to prevent Republicans from blocking Biden's initiatives. White House Communications Director Kate Bedingfield made clear on Sunday that the president is not calling for ending the filibuster. Biden previously served in the Senate for...