WASHINGTON — The future of the Senate filibuster is increasingly in doubt, even though Republicans have yet to use one to block a single Democratic bill in the Biden era.

While the real showdown lies ahead, President Biden’s endorsement this week of a change in the rules governing the Senate’s signature procedural weapon represented a major shift in the political dynamic. Almost overnight, the prospect of Democrats taking action to weaken the minority party’s power to stall legislation has shifted from a far-off theoretical question to a fast-moving push with presidential buy-in.