The filibuster is not in the constitution. Currently, the senate has adopted a rule by which debate may be closed with 60 votes. So if one side has 41 votes they may, by convention, ‘filibuster’ (which entails nothing but saying ‘we filibuster’) and the bill fails. The majority doesn’t have to accept this though, and can force the other side to literally filibuster, by ‘debating’ continuously, so as to prevent a vote. It is unlikely that this could be maintained forever, especially when the issue to be voted on enjoyed popular support. As a famous example, see Strom Thurmond’s filibuster of the Civil Rights Act of 1957.
I did some research and it’s more egregious than I thought: Senate rules can be changed by a simple majority, and unlimited debate is just a rule— prior to 1806 there was limited debate. So the democrats could just completely eliminate the filibuster, in any form, and pass what they choose, so long as they have 51 votes, or 50 and the Vice President.
https://en.m.wikipedia.org/wiki/Strom_Thurmond_filibuster_of...